BIOLOGY 

LIBRARY 

G 


NATURE  AND 
DEVELOPMENT  OF  PLANTS 

CURTIS 


NATURE  AND          :  : 
DEVELOPMENT  OF  PLANTS 


BY 


CARLTON  C.  CURTIS,  A.M.,  PH.D. 

Professor  in  Botany  in  Columbia  University 


SEVENTH  EDITION-REVISED 


NEW  YORK 
HENRY  HOLT  AND  COMPANY 

1918 


BIOLOGY 

LIBRARY 

G 


Copyrighted  1907,  1910,  1914.  IQIS.  1916.  1917,  1918  by 
HENRY  HOLT  AND  COMPANY 


PRESS  OF 

THE  NEW  ERA  PRINTING  COMPANY 
LANCASTER,  PA. 


PREFACE 


In  the  present  volume  an  attempt  has  been  made  to  present 
the  more  important  aspects  of  botanical  science,  not  only  as 
they  are  related  to  pure  biological  science,  but  also  as  they  are 
connected  and  associated  with  our  daily  life  and  interests.  The 
general  reader  will  find  here  a  discussion  of  the  laws  and  prin- 
ciples that  control  and  direct  plant  life  and  of  the  importance 
of  this  life.  The  author  also  has  had  in  mind  the  difficulties 
that  the  student  experiences  in  entering  a  new  field.  To  make 
the  subject  more  approachable,  many  of  the  technical  terms  have 
been  eliminated  and  an  easily  comprehended  terminology  has 
been  maintained  throughout  the  book.  The  text  is  designed 
especially  for  the  benefit  of  the  student  who  is  beginning  the 
subject.  We  trust  that  a  study  of  it  will  bring  him  to  the  class 
room  prepared  for  a  discussion  of  the  topics  and  we  also  trust 
that  this  work  of  preparation  will  tax  him  to  the  full  measure 
of  his  intellectual  capacity.  The  author  is  old  fashioned  in  his 
ideas  of  education.  Work  that  simply  entertains  or  imparts 
information  and  that  does  not  create  the  sufferings  associated 
with  mental  effort  can  be  of  little  permanent  value  or  make  for 
any  considerable  development.  Finally  the  book  will  fall  short 
of  its  real  purpose  unless  used  in  the  laboratory  in  connection 
with  the  plants  themselves.  Detailed  directions  are  not  given 
for  the  laboratory  work — these  of  necessity  must  vary  with  the 
instructor  and  with  the  locality — but  the  text  gives  the  student 
sufficient  guidance  and  understanding  to  enable  him  to  find  out 
for  himself  the  principles  and  facts  in  the  material  to  which  he 
has  been  directed. 

Acknowledgments  are  due  to  Professors  C.  B.  Atwell,  J.  H. 
Schaffner,  F.  D.  Kern,  Ira  D.  Cardiff,  Jean  Broadhurst,  C.  A. 
Darling,  A.  H.  Chivers,  B.  O.  Dodge,  R.  C.  Benedict,  G.  W. 
Martin  and  several  others  for  valuable  suggestions,  in  the  prepa- 


vi  PREFACE 

ration  of  various  portions  of  the  text.  The  author  desires  to 
express  his  especial  indebtedness  to  his  good  friend,  Mr.  H.  O. 
Hanson,  for  the  preparation  of  many  of  the  illustrations;  to  Mr. 
W.  A.  Andrews,  for  microscopical  studies  from  which  drawings 
have  been  made,  and  to  Miss  Mary  L.  Williams,  for  the  execu- 
tion of  many  of  the  studies.  Special  mention  is  also  made  of  the 
author's  indebtedness  to  the  assistance  and  collaboration  of  his 
former  pupils  and  present  associates,  Drs.  E.  Altenburg  and  H. 
J.  Mullen 

CARLTON  C.  CURTIS. 
COLUMBIA  UNIVERSITY, 
June.  1918. 


CONTENTS 

PAGE 

INTRODUCTION i 

PART  I 
NATURE  OF  PLANTS 

I.    THE  LEAF 7 

II.    THE  ROOT 46 

III.  THE  STEM 71 

IV.  THE  FLOWER,  FRUIT  AND  SEEDLING 115 

PART  II 
THE  DEVELOPMENT  OF  PLANTS 

V.    CLASSIFICATION  OF  PLANTS 146 

VI.    THALLOPHYTA 149 

VII.    BRYOPHYTA 269 

VIII.    PTERIDOPHYTA 311 

IX.    SPERMATOPHYTA  . 352 

X.    ANGIOSPERMAE  (SPERMATOPHYTA  CONCLUDED) 378 


Vll 


NATURE  AND  DEVELOPMENT  OF  PUNTS 


INTRODUCTION 

i.  The  Nature  of  the  Plant. — It  is  a  familiar  fact  that  most 
plants  have  stems,  leaves  and  roots  and  that  these  organs  are 
variously  modified.  Why  have  such  organs  been  developed? 
Why  do  they  assume  such  varied  forms  and  arrangements?  How 
does  it  come  about  that  the  stem  reaches  up  in  the  air  and  that 
the  branches  and  leaves  are  arranged  in  a  very  definite  order 
while  the  root  penetrates  the  soil  and  grows  toward  moisture  and 
other  soil  material?  Is  a  plant,  like  an  animal,  conscious  of  its 
surroundings,  seeking  suitable  foods  and  avoiding  unfavorable 
conditions?  It  is  evident  to  the  casual  observer  that  the  plant 
is  sensitive  to  its  surroundings;  some  plant  organs  bend  toward 
a  light  that  we  recognize  with  difficulty  or  react  to  a  touch  that 
we  cannot  appreciate.  Has  the  plant  a  nervous  system  and 
faculties  like  ourselves  that  enable  it  to  adjust  itself  to  its  sur- 
roundings? To  answer  these  questions  it  will  be  necessary  to 


FIG.  i.  Diagram  of  a  cell  showing  its  three  dimensions;  mesh-wqrk  of 
granular  cytoplasm  which  incloses  colorless  cell  sap,  the  vacuoles,  and  a  denser 
dark  body,  the  nucleus. 

consider  the  nature  of  the  living  substance  of  the  plant.  The 
plant  body  is  composed  of  cells  that  are  like  boxes  of  living 
matter  having  length,  breadth,  and  thickness  (Fig.  i).  While 
the  cells  assume  various  forms  and  perform  different  work  or 

i 


i  THE  LIVING  SUBSTANCE  OF  PLANTS 

functions,  they  all  possess  cell  walls  which  contain  during  the 
life  of  the  cell  a  substance  termed  protoplasm.  This  substance 
resembles  somewhat  the  white  of  an  egg,  being  viscid  and  rather 
tenacious.  The  protoplasm  is  not  of  uniform  consistency.  The 
larger  portion  of  it  is  finely  granular  and  is  called  the  cytoplasm 
while  a  denser  rounded  part  is  termed  the  nucleus.  Other  bodies, 
plastids,  also  denser  than  the  cytoplasm  are  of  common  occur- 
rence. Such  plastids  as  contain  a  green  pigment,  chlorophyll,  are 
called  chloroplastids  or  chloroplasts,  and  these  produce  the  green 


FIG.  2.  Structure  of  the  cell:  A,  cell  ditch-moss,  Pkilotria;  n,  nucleus, 
v,  vacuole,  ch,  chloroplast.  B,  cell  of  carrot;  ch,  yellowish  chromoplasts,  nt 
nucleus.  C,  reddish  chromoplasts  from  rose  hip.  D,  cells  of  Begonia;  s, 
starch  grains,  c,  crystals  of  lime.  E,  leucoplast,  /,  of  potato  forming  a  starch 
grain. 


color  of  the  vegetation  (Fig.  2,  A).  Still  other  plastids,  the 
chromoplasts,  contain  red  or  yellow  pigments  that  give  the  color 
to  many  fruits  and  flowers,  as  the  tomato,  rose-hip,  squash,  nas- 
turtium, etc.  (Fig.  2,  B,  C).  Colorless  plastids,  leucoplasts,  often 


INTRODUCTION  3 

occur  in  cells  hidden  from  the  light.  One  group  of  these  leuco- 
plasts  forms  the  starch  that  appears  in  underground  storage 
organs  as  in  the  potato  (Fig.  2,  E).  Usually  spaces  or  vacuoles, 
that  appear  as  cavities  also  occur  in  the  granular  cytoplasm 
(Fig.  2,  A,  v).  In  reality  they  are  filled  with  watery  solutions  of 
various  substances,  the  so-called  cell  sap.  Many  other  structures 
(Fig.  2,  D)  appear  in  the  protoplasm  to  which  attention  will  be 
called  later.  It  is  well  to  remember  that  these  visible  structures 
do  not  represent  the  real  composition  of  the  protoplasm.  A 
variety  of  units  beyond  the  range  of  visibility,  grow,  multiply, 
and  build  up  the  various  structures  which  we  recognize  as  con- 
stituting the  protoplasm.  The  cell  walls  make  up  the  body  of 
the  plant  and  give  stability  to  its  various  organs  but  the  living 
part  of  the  cell  is  the  protoplasm.  This  substance  reaches 
through  delicate  pores  in  the  cell  wall  to  the  protoplasm  of 
adjacent  cells  so  that  all  the  living  substance  of  the  entire  plant 
body  is  in  contact  and  forms  one  united  mass. 

2.  The  Nature  of  the  Living  Substance  of  the  Plant. — The  pro- 
toplasm possesses  most  remarkable  powers.  It  can  absorb  vari- 
ous fluids  and  gases,  decompose  them  into  simple  elements,  re- 
unite them  into  foods  or  discharge  from  the  cell  such  substances 
as  are  not  required.  Furthermore  the  protoplasm  effects  those 
changes  termed  growth  by  transforming  the  foods  into  the  sub- 
stances that  compose  the  cell  walls  and  other  parts  of  the  cell* 
as  the  protoplasm  itself.  How  are  these  changes  brought  about? 
Every  substance  is  composed  of  elements.  Water  consists  of 
two  elements,  hydrogen  and  oxygen.  The  elements  that  compose 
any  substance  are  held  together  with  great  energy,  owing  to  their 
mutual  attraction  for  one  another.  When  two  different  sub- 
stances are  brought  together,  it  may  happen  that  the  attraction 
of  certain  elements  of  one  substance  is  greater  for  one  or  more 
of  the  elements  of  the  other  substance  than  for  its  own  elements. 
The  result  is,  that  the  elements  will  be  torn  away  from  their 
respective  substances  and  united  into  new  combinations.  We 
say  that  a  decomposition  and  a  re-combination  has  been  effected, 
or,  that  a  chemical  reaction  has  occurred.  We  see  an  illustration 
of  these  chemical  changes  when  iron-ore  is  heated  with  charcoal. 


4  NATURE   OF  THE   LIVING   SUBSTANCE 

The  ore  is  composed  of  the  elements  iron  and  oxygen  and  the 
charcoal  consists  of  carbon.  In  the  presence  of  heat  the  carbon 
has  a  stronger  attraction  for  the  oxygen  than  the  iron  has.  Con- 
sequently the  oxygen  is  drawn  away  from  the  iron  and  unites 
with  the  carbon,  forming  a  new  combination  of  carbon  and  oxy- 
gen, and  leaving  the  iron  free.  This  decomposition  and  re-com- 
bination is  only  a  part  of  the  chemical  change  that  takes  place, 
but  it  will  serve  to  illustrate  the  nature  of  many  chemical  re- 
actions. The  energy  that  was  required  to  hold  the  oxygen  and 
iron  together  begins  to  be  set  free  as  soon  as  the  decomposition 
starts  and  it  contributes  to  the  rise  of  temperature  during  the 
reaction.  The  living  substance  of  the  plant  is  composed  of  a 
great  variety  of  chemical  substances  that  are  very  readily  de- 
composed by  gases  and  other  substances  which  it  absorbs.  Light, 
heat,  gravitation  and  moisture  are  even  more  powerful  in  pro- 
ducing these  changes.  These  substances  and  forces  are  often 
referred  to  as  stimuli  (sing,  stimulus)  because  they  start  the 
chemical  changes  referred  to  above.  So  the  various  substances 
and  forces  in  nature  stimulate  or  cause  chemical  changes  in  the 
living  substance  of  the  plant  and  the  energy  thus  set  free  is  used 
by  the  protoplasm  in  the  performance  of  its  work.  The  cells  of 
a  seed  have  no  power  of  their  own  to  grow  or  perform  any  duty. 
It  is  not  until  a  certain  amount  of  heat,  moisture,  gases  or  other 
stimulating  forces  have  acted  upon  the  protoplasm  or  substances 
contained  in  it  and  so  aroused  chemical  changes  that  it  is  fur- 
nished with  the  requisite  energy  to  begin  growth.  Furthermore, 
these  forces  exert  a  very  definite  influence  upon  the  protoplasm 
and  cause  it  to  accomplish  very  definite  results.  This  is  due  to 
the  fact  that  not  all  portions  of  the  protoplasm  of  the  plant  body 
are  equally  influenced  by,  or  we  may  say  equally  sensitive  to 
heat,  light,  and  other  forces  and  consequently  the  various  parts 
of  the  plants  do  not  respond  alike.  The  force  of  gravity,  for 
example,  acts  upon  some  stems  and  the  living  substance  is  in- 
fluenced by  this  stimulus  so  that  a  growth  is  aroused  that  bends 
the  stem  into  an  upright  position.  The  protoplasm  in  the  cells 
of  the  root,  however,  is  so  constituted  that  the  stimulus  of  gravity 
causes  a  growth  that  bends  the  root  down  into  the  soil.  Light 


NATURE  OF  PLANTS  5 

stimulates  the  mustard  plant  in  such  a  way  that  the  stem  grows 
toward  the  light  and  the  root  away  from  it.  So  the  living  sub- 
stance of  the  plant  while  forming  one  united  whole  varies  in  its 
sensitiveness  in  various  parts  of  the  plant.  In  this  respect  the 
various  parts  may  be  compared  to  the  receiving  stations  of  a 
wireless  telegraphy  system.  The  receiving  stations  may  have 
instruments  sensitive  to  certain  intensities  of  electrical  currents 
and  other  currents  do  not  affect  them.  So  the  various  organs  of 
the  plant  are  sensitive,  each  in  its  own  peculiar  way,  to  certain 
stimuli.  As  a  result  of  this  adjustment  the  various  parts  of  the 
plant,  being  attuned  or  sensitive  to  certain  forces,  are  led  or 
induced  to  grow  so  that  they  come  into  the  most  helpful  and 
beneficial  relation  to  these  forces.  Thus  the  leaves,  branches, 
and  roots  are  stimulated  to  grow  and  develop  so  that  each  part 
becomes  properly  related  to  light,  moisture,  gravity,  etc.  The 
broad  blades  of  the  majority  of  leaves  are  exposed  to  the  direct 
rays  of  light.  This  arrangement  is  of  the  greatest  benefit  be- 
cause they  are  using  the  light  in  the  manufacture  of  foods.  This 
position  is  assumed,  however,  because  they  are  sensitive  to  light, 
gravitation  and  other  forces  which  direct  and  cause  the  arrange- 
ment. This  is  the  most  noteworthy  feature  about  the  sensitive- 
ness of  the  protoplasm.  Forces  stimulate  to  growth  in  such  a 
way  that  the  results  are  helpful  and  the  greatest  good  comes  to 
the  plant.  In  other  words  the  reactions  are  purposive.  These 
growths  are  so  elaborate  and  beneficial  that  they  often  appear  as 
the  result  of  reason  and  will.  The  bending  of  the  root  into  the 
soil  brings  it  into  contact  with  water  and  other  foods;  the  tendril 
coils  about  a  branch  and  binds  the  plant  firmly  to  the  support. 
The  plant,  however,  does  not  direct  these  adjustments.  These 
reactions  do  not  involve  consciousness.  These  movements  and 
all  others  are  absolutely  directed  and  controlled  by  the  various 
forces  which  act  upon  the  sensitive  protoplasm.  A  definite  re- 
action follows  a  stimulation  which  the  plant  has  no  power  to 
alter  or  control.  The  root  will  bend  down  into  a  dish  of  mercury 
with  the  same  energy  and  directness  as  that  with  which  it  pene- 
trates the  soil,  and  the  tendril  will  clasp  your  finger  or  a  branch 
of  its  own  body  as  firmly  as  a  serviceable  support. 


6  AIM   OF  THE   STUDY 

We  shall  now  proceed  to  study  the  plant  from  this  standpoint, 
giving  attention  to  the  fitness  or  adaptation  of  the  leaf,  root  and 
stem,  to  the  conditions  under  which  the  plant  lives,  and  to  the 
work  which  it  performs;  and  finally  we  will  be  interested  to 
learn  how  the  seed  is  formed  and  how  the  plant  lives  from  year 
to  year. 


PART  I 
THE  NATURE  OF  PLANTS 


CHAPTER   I 

THE  LEAF 

3.  The  Work  of  the  Leaf. — Each  plant  has  a  character  or  a 
personality  of  its  own.  What  is  it  that  gives  this  individuality 
to  the  plant?  When  the  work  that  it  performs  is  understood  it 
will  be  seen  that  this  character  is  very  largely  due  to  the  adapta- 
tion of  the  leaves  to  the  performance  of  certain  duties.  The 


6 


FIG.  3.  Forms  of  leaves:  A,  leaf  of  white  birch  with  netted  veins — p, 
petiole;  b,  blade.  B,  leaf  of  Solomon's  seal  with  parallel  venation  and  blade 
clasping  stem  without  petiole. 

leaves  bring  the  plant  into  harmony  with  its  surroundings  and 
give  to  it  a  subtle  individuality  owing  to  the  perfection  of  their 
arrangements,  structures  and  forms  for  the  work  in  hand,  The 

7 


8  THE  WORK  OF  THE   LEAF 

more  important  work  performed  by  the  leaves  is  the  construction 
of  foods,  the  giving  off  of  water  or  transpiration,  and  breathing 
or  respiration.  The  magnitude  of  this  work  far  exceeds  the 
energy  expended  in  all  the  industries  of  the  world.  The  leaves, 
however,  accomplish  this  work  so  quietly  and  economically  that 
most  people  are  scarcely  conscious  of  it. 

In  order  to  understand  the  purpose  of  the  petiole  and  green 
blade  (Fig.  3)  or  the  significance  of  the  various  forms  and 
arrangements  of  leaves  it  will  be  necessary  to  examine  the  struc- 
ture of  the  leaf  and  see  the  character  of  the  apparatus  that  is 
used  in  the  performance  of  its  work.  The  blades  of  the  majority 


FIG.  4.     Blade  of  lilac  leaf  cut  across,  showing  the  more  compact  arrange- 
ment of  cells  upon  the  upper  side  of  the  leaf. 

of  leaves  are  flat.  A  section  through  such  a  blade,  cut  as  in  Fig. 
4  so  that  we  can  look  into  the  end  of  the  blade,  shows  that  the 
leaf  is  composed  of  a  complicated  arrangement  of  cells.  Fig.  5 
is  a  greatly  enlarged  view  of  Fig.  4  taken  at  X. 

4.  The  Epidermis. — It  is  now  seen  that  a  layer  of  compact  cells 
(Fig.  5,  e)  surrounds  the  leaf  on  all  sides.  In  fact  such  a  layer 
covers  all  parts  of  the  plant  body.  This  layer  of  cells,  the  epi- 
dermis, is  provided  with  minute  openings  or  stomata  (sing, 
stoma),  Fig.  5,  s.  The  stomata  are  especially  abundant  in  the 
epidermis  on  the  under  surface  of  the  leaf  and  often  quite  lacking 
from  the  upper  surface.  A  better  idea  of  these  openings  and  of 
the  epidermal  cells  may  be  gained  by  stripping  off  the  epidermis 


NATURE   OF   PLANTS 


FIG.  5.     Greatly  enlarged  view  of  region  X  in  Fig.  4:  e,  epidermis;  s,  stbmafN 
p,  palisade  mesophyll;  ch,  chloroplast;  sp,  spongy  mesophyll;  i,  intercellular 
spaces;  v,  small  vein  cut  across;  /,  end  of  vein  seen  from  the  side,  consisting 
of  elongated  and  banded  cells  (tracheids);  col,  collecting  cells  that  transfer    / 
material  to  and  from  the  palisade  cells, — H.  O.  Hanson. 


FIG.  6.     Surface  view  of  epidermis  of  lilac  leaf:  g,  guard  cell;  s,  stoma;  ch, 
chloroplasts;  n,  nucleus. — H.  O.  Hanson. 


io  STRUCTURE   OF  THE   LEAF 

from  such  leaves  as  the  live-for-ever,  blue  flag  or  hyacinth.  The 
cells  of  the  epidermis  are  very  compactly  put  together  and  may  be 
stripped  off  by  means  of  a  penknife  as  a  thin  white  skin.  Ex- 
amined under  a  microscope,  such  preparations  show  the  stomata 
as  minute  openings  guarded  by  two  rather  lens-shaped  cells, 
termed  the  guard  cells  (Fig.  6).  Chloroplasts  usually  occur 
only  in  the  guard  cells,  the  other  epidermal  cells  having  a  watery, 
colorless  cell  content.  In  addition  to  these  features,  Fig.  7  shows 
that  the  outer  portion  of  the  walls  of  the  epidermal  cells  is  modi- 
fied so  as  to  form  a  delicate,  skin-like  layer  over  the  outer  sur- 


FIG.  7.  FIG.  8. 

FIG.  7.     Section  across  a  stoma  shown  in  Fig.  6:  s,  stoma;  g,  guard  cell: 
c,  cuticle;  a,  air  chamber.  —  H.  O.  Hanson. 

FIG.  8.     Hairs  from  the  epidermis  of  squash  leaf. 

face  of  the  epidermis.  This  part  of  the  cell  wall  is  called  the 
cuticle.  It  contains  a  fatty  substance,  cutin,  that  renders  the 
epidermis  nearly  impervious  to  gases  and  fluids,  so  that  the  leaf 
is  entirely  surrounded  by  a  water-  and  gas-proof  coat.  In  many 
leaves  hairs  of  various  forms  grow  out  from  the  epidermal  cells 
(Fig.  8). 

5.  The  Mesophyll.  —  The  cells  within  the  epidermis  have,  for 
the  most  parts,  a  greenish  color  due  to  numerous  green  bodies, 
chloroplasts,  contained  in  the  cells  (Fig.  5,  ck).  These  green 
cells  of  the  leaf,  termed  the  mesophyll,  consist  of  one  or  more 
layers  of  rather  elongated  and  compact  cells  on  the  upper  side 
of  the  leaf  (the  palisade  mesophyll,  Fig.  5,  p),  and  below  they 

w- 


NATURE   OF   PLANTS  11 

are  joined  to  rather  irregular  and  loosely  arranged  cells  (the 
spongy  mesophyll,  Fig.  5,  sp).  Numerous  air  spaces  extend  in 
all  directions  through  this  spongy  mass  of  cells  reaching  up  to 
and  between  the  palisade  cells  and  down  to  the  stomata  (Fig. 
5,  i).  Bundles  of  small  cells  also  appear  here  and  there  between 
the  palisade  and  spongy  mesophyll  (Fig.  5,  v,  I).  These  struc- 
tures are  the  so-called  veins  that  appear  as  fine  lines  in  the 
majority  of  leaves  (Fig.  3).  These  veins  or  vascular  bundles 
branch  again  and  again  and  so  extend  to  all  parts  of  the  leaf. 
In  Fig.  5,  v  one  of  the  veins  has  been  cut  square  across  while 
at  /  the  end  of  a  small  vein  is  shown,  having  been  cut  in  half 
lengthwise.  The  walls  of  the  mesophyll  cells  are  smooth,  thin, 
and  elastic  and  composed  of  a  substance  called  cellulose.  Thin 
places  or  pores  are  often  formed  in  the  walls  which  appear  as 
openings  owing  to  the  extreme  thinness  of  the  wall  at  such  places. 
The  term  parenchyma  is  applied  to  all  cells  possessing  the  char- 
acter of  walls  noted  in  the  mesophyll  cells.  In  the  vascular 
bundle,  however,  a  portion  of  the  cells  have  thicker  walls  which 
are  more  rigid,  hard  and  woody.  Such  cells  are  often  referred  to 
as  prosenchyma. 

What  is  the  significance  of  the  peculiar  and  varied  arrange- 
ments of  the  cells  in  the  leaf?  We  naturally  anticipate  that  these 
different  structures  are  adapted  to  performance  of  special  duties. 
Attention  will  first  be  directed  to  the  work  performed  by  the 
green  portion  of  the  leaf,  the  mesophyll.  Similar  cells  containing 
chloroplasts  are  of  common  occurrence  in  various  parts  of  the 
plant,  being  often  found  in  stems  and  unripe  fruits.  In  the  leaf 
such  a  tissue  is  termed  mesophyll  but  a  more  general  term,  Chlor- 
enchyma,  includes  all  chlorophyll-bearing  tissue  wherever  found. 

6.  The  First  Function  of  the  Leaf,  Photosynthesis. — Chlor- 
enchyma  is  the  most  important  tissue  in  the  plant  since  it  has 
for  its  special  function  or  work  the  construction  of  carbohydrates, 
such  as  sugars  and  starches,  which  are  among  the  most  impor- 
tant foods  of  the  plant.  The  construction  of  these  carbohydrates 
by  the  chlorenchyma  is  called  photosynthesis.  This  work  easily 
ranks  as  the  leading  wonder  of  the  world  because  its  processes 
are  so  complicated  and  of  such  magnitude  and  importance  in  the 


12  NATURE   OF   CARBOHYDRATES 

economy  of  this  world  of  ours.  It  will  be  seen  that  this  tissue 
not  only  keeps  the  air  in  a  wholesome  condition  for  breathing 
and  so  makes  life  possible  upon  the  earth  but  that  all  animal  life 
is  absolutely  dependent,  either  directly  or  indirectly,  for  food 
upon  the  products  built  up  by  the  chlorenchyma.  Let  us  see 
how  these  foods  are  formed.  •  Various  substances  are  absorbed 
by  the  roots,  and  the  vascular  bundles,  which  extend  throughout 
the  plant  body,  transfer  them  to  the  chlorenchyma.  This  ab- 
sorbed material  is  in  large  part  water,  but  various  mineral  sub- 
stances are  dissolved  in  it.  Various  gases,  as  carbon  dioxide, 
also  find  their  way  through  the  stoma  to  the  chlorenchyma.  All 
of  these  absorbed  substances  are  composed  of  elements.  Thus 
water  consists  of  two  elements,  hydrogen  and  oxygen,  in  the  pro- 
portion of  two  parts  of  hydrogen  to  one  part  oxygen.  This  com- 
position is  indicated  by  the  abbreviation,  H2O.  So  the  gas, 
carbon  dioxide,  contains  one  part  of  carbon  and  two  parts  of  oxy- 
gen, indicated  thus  CO2.  Chloroplasts  have  the  power  under 
suitable  conditions  of  decomposing  or  tearing  apart  these  com- 
pounds and  of  reuniting  their  elements  into  new  compounds  that 
are  quite  different  from  the  original  ones.  In  this  way  CO2 
and  H2O  are  decomposed  and  the  elements  reunited  into  complex 
compounds  or  foods  consisting  of  carbon,  hydrogen  and  oxygen. 
Such  complex  foods  as  cane  sugar,  consisting  of  twelve  parts  of 
carbon,  twenty-two  of  hydrogen,  and  eleven  of  oxygen  are  formed 
in  this  way.  This  very  common  food  is  expressed  by  the  formula 
Ci2H22On.  So  also  other  foods  are  formed,  as  grape  sugar 
CeHiuOfi  and  starch  usually  indicated  as  some  multiple  of  CeHioOs. 
The  cellulose  walls  of  the  parenchyma  cells  have  the  same  com- 
position as  starch.  It  will  be  seen  that  all  these  foods  have 
carbon  united  to  hydrogen  and  oxygen  in  the  same  proportion 
as  it  exists  in  water.  Cane  sugar  has  carbon  united  to  eleven 
times  H2O,  and  starch  five  times  H2O.  It  was  supposed  that 
these  substances  were  a  combination  of  carbon  and  H2O.  For 
this  reason  they  were  called  carbohydrates.  This  name  is  still 
applied  to  these  foods  although  it  is  known  that  the  base  is  not 
H2O  but  the  hydroxyl  OH.  The  actual  changes  that  are  effected 
in  the  formation  of  these  various  carbohydrates  are  not  known. 


NATURE   OF   PLANTS  13 

It  has  been  suggested  that  CO2  and  H2O  enter  into  solution  form- 
ing carbonic  acid  (H2CO3).  This  change  is  represented  by  the 
equation,  CO2  +  H2O  =  H2CO3.  By  giving  off  one  part  of 
oxygen  this  acid  is  changed  to  formic  acid  (CH2O2),  i.  e.,  H2COa 
=  CH2O2  +  O.  A  similar  decomposition  is  effected  in  the  formic 
acid  which  results  in  the  formation  of  formaldehyde  (CH2O), 
i.  e.,  CH2O2  =  CH2O  +  O.  The  two  parts  of  oxygen  pass 
from  the  cells  and  escape  through  the  stomata  as  a  gas.  In 
the  presence  of  certain  alkalies  and  acids,  formaldehyde  increases 
the  number  of  elements  which  compose  it,  thus  six  times  CH2O 
would  give  C6Hi2O6  or  grape  sugar.  This  is  but  a  theory  based 
upon  the  facts  that  CO2  and  H2O  do  unite  in  nature  to  form 
H2CO3  and  that  this  acid  may  decompose  as  stated  above,  and 
finally  that  formaldehyde  can  be  detected  in  plant  cells  and  it 
is  possible  to  produce  sugar  from  this  substance  by  treating  it 
with  alkalies  and  acids.  All  that  can  be  definitely  stated  about 
the  changes  going  on  in  the  leaf,  during  the  formation  of  the 
carbohydrates  relates  solely  to  the  beginning  and  end  of  the 
process.  Water  and  carbon  dioxide  enter  the  chlorenchyma 
cells.  A  series  of  changes  follows,  the  nature  of  which  can  only 
be  conjectured.  Finally,  as  the  end  result,  sugar  appears  in 
the  cells  and  oxygen  is  set  free,  escaping  as  a  gas. 

These  complex  changes  are  brought  about  in  the  leaf  in  so 
subtle  a  way  that  we  are  not  conscious  of  them  or  of  the  great 
amount  of  energy  that  is  required  to  effect  them.  It  requires 
the  energy  expressed  by  a  temperature  of  1300°  C.  to  decompose 
COa  into  its  elements.  Where  does  the  plant  obtain  the  energy 
to  bring  about  the  decompositions  and  recomposition?  This 
work  is  accomplished  by  the  energy  of  the  sunlight  acting  upon 
the  chloroplasts.  The  chloroplasts  of  the  seed  plants  are  minute, 
rather  lens-shaped  grains,  and  increase  in  number  by  the  division 
of  the  plastid  into  two  equal  parts  (Fig.  9).  These  bodies  are 
denser  portions  of  the  cytoplasm  and  like  it  are  nearly  colorless. 
In  the  presence  of  light  a  green,  oily  substance,  chlorophyll,  is 
formed  in  the  plastids,  thus  producing  their  green  color.  Chloro- 
phyll is  rarely  formed  except  in  the  presence  of  light.  Plants 
grown  in  cellars  or  in  the  dark  are  of  a  pale  color  owing  to  the 


14  NATURE  OF  CHLOROPHYLL 

absence  of  chlorophyll  in  the  plastids.  Similarly  green  plants 
lose  their  chlorophyll  when  placed  in  the  dark.  This  is  the 
principle  employed  in  the  blanching  of  celery.  The  stalks  are 
surrounded  by  earth  or  the  light  is  excluded  from  them  by  some 
other  means;  chlorophyll  disappears  from  the  plastids  and  fails 
to  develop  in  the  new  cells  that  are  formed  in  the  dark.  Plants 
in  which  the  chlorophyll  is  not  developed  are  said  to  be  etiolated . 
The  plastids  are  in  the  cells,  however,  for  if  an  etiolated  seed- 
ling is  removed  from  the  dark  to  the  light  the  chlorophyll  will 
begin  to  appear  in  a  few  hours.  The  greening  of  potatoes  when 
removed  from  the  ground  and  left  in  a  strong  light  is  a  familiar 
example  of  this.  It  is  evident  that  the  formation  of  chlorophyll 
is  usually  closely  connected  with  the  action  of  light.  Another 
important  point  regarding  the  influence  of  light  is  the  fact  that 
such  a  food  as  starch  is  not  usually  found  in  the  leaf  in  the 
absence  of  light.  This  may  be  easily  verified  by  cutting  the 
initials  of  your  name  in  rather  large  letters  in  a  strip  of  black 
paper  or  smooth  tin-foil  and  fastening  the  strip  in  the  early 
morning  to  the  upper  surface  of  a  well-sunned  leaf  of  a  starch 
forming  plant.  After  a  few  hours  remove  the  leaf  from  the  plant 
and  place  it  in  alcohol.  This  will  dissolve  the  chlorophyll  and 
the  leaf  will  become  quite  white  in  a  few  hours.  Now  place  the 
leaf  in  a  tincture  of  iodine  which  turns  starch  to  a  blue  or  blue- 
black  color.  Wherever  the  light  was  excluded  from  the  leaf 
there  is  only  a  pale  yellow  color  but  all  portions  of  the  sunned 
leaf  show  an  abundance  of  starch  as  revealed  by  the  blue  or  blue- 
black  coloration.  Consequently  the  letters  appear  black  on  a 
yellow  background  which  marks  the  limits  of  the  strip  of  paper. 
No  starch  can  be  detected  as  a  rule  in  leaves  taken  from  plants 
that  have  been  growing  in  the  dark  for  twenty-four  hours. 
Plastids  that  do  not  contain  chlorophyll,  examples  of  which  are 
found  in  the  cells  forming  the  white  bands  or  blotches  in  many 
variegated  plants,  do  not  form  starch  in  the  light.  It  is  evident 
from  these  facts  that  the  chlorophyll  and  light  co-operate  in 
some  way  in  the  formation  of  starch.  It  is  known  that  chloro- 
phyll absorbs  a  certain  portion  of  the  sunlight.  If  a  beam  of 
sunlight  is  caused  to  pass  through  a  prism  it  is  separated  into 


NATURE   OF   PLANTS  15 

seven  colors,  i.  e,,  red,  orange,  yellow,  green,  blue,  indigo  and 
violet.  When,  however,  a  beam  first  passes  through  an  alcoholic 
solution  of  chlorophyll  or  through  a  green  leaf  a  portion  of  certain 
colors  and  all  of  other  colors  do  not  appear.  The  chlorophyll  has 
taken  up  or  absorbed  certain  of  the  light  rays  especially  portions 
of  the  red,  blue  and  violet.  It  is  reasonable  to  suppose  that  the 
energy  in  these  rays  is  utilized  in  part  by  the  plastids  in  effecting 
the  changes  noted  above  in  the  formation  of  foods.  The  sun 
gives  to  the  earth  great  quantities  of  energy  in  the  form  of  light. 
A  portion  of  this  light  is  taken  up  by  the  chlorophyll,  and  the 
plastid  uses  the  energy  of  the  light  to  bring  about  the  recombi- 
nation of  CO2  and  H20  and  the  formation  of  sugars,  starches, 
and  other  carbohydrates.  Because  of  the  importance  of  the 
sunlight  in  the  formation  of  carbohydrates  this  process  is  called 
photosynthesis.  This  term  means  the  putting  together  by  means 
of  light. 

7.  The  Magnitude  of  the  Work  of  Photosynthesis. — Carbon  is 
an  important  element  in  the  composition  of  plant  foods  and  also 
in  the  walls  of  the  cells.  It  forms  one  half  of  the  dry  weight  of 
the  plant.  All  the  carbon  appearing  in  the  plant  is  derived  solely 
from  the  CO2  in  the  air,  of  which  it  forms  a  very  small  part,  only 
about  three  parts  in  ten  thousand.  Furthermore  the  carbon 
comprises  only  3/11  by  weight  of  the  CO2,  8/n  being  oxygen. 
A  square  meter  of  leaf  surface,  however,  can  withdraw  all  the 
CO2  from  2500  liters  of  air  in  one  hour.  Such  a  volume  of  air 
would  be  a  space  one  meter  square  and  2^  meters  high.  This 
would  furnish  sufficient  carbon  for  the  construction  of  one  gram 
of  starch.  In  this  way  such  large  quantities  of  CO2  are  drawn 
into  the  chlorenchyma  cells  as  to  make  possible  each  year  the 
harvest  and  the  renewal  of  vegetation  of  the  earth.  In  the 
United  States  alone  in  this  way  there  was  in  1917  built  up  over 
5400  millions  of  pounds  of  cotton,  650  million  bushels  of  wheat, 
1587  million  bushels  of  oats,  etc.,  representing  a  value  for  all 
crops  of  ii  billion  dollars.  Our  leading  crop  is  corn  which 
amounted  to  over  3  billion  bushels  and  valued  at  about  4  billion 
dollars.  The.  gold  and  silver  coin  and  bullion  of  the  country  are 
not  of  greater  value.  This  corn  came  up  and  matured  in  120 


1 6  NATURE   OF   PROTEIDS 

days,  consequently  these  coin  plants  were  manufacturing  food 
at  the  rate  of  over  33  million  a  day.  These  figures  represent  only 
a  small  portion  of  the  work  performed,  since  the  grain  is  but  a 
part  of  the  plant,  and  furthermore  the  larger  portion  of  the  land 
is  covered  with  forests  and  other  forms  of  vegetation.  The  fact 
must  not  be  overlooked  that  this  process  of  photosynthesis  is  of 
vital  importance  to  our  welfare  in  another  way.  Owing  to  the 
large  volumes  of  CO2  that  are  constantly  formed  by  fires  and  the 
respiration  of  animals  such  an  excess  of  this  gas  would  accumu- 
late in  the  atmosphere  that  all  animal  life  would  eventually  cease 
were  it  not  for  its  absorption  in  photosynthesis.  So  balanced 
are  the  rates  of  formation  and  absorption,  however,  that  the 
percentage  existing  in  the  air  does  not  materially  vary. 

8.  The  Construction  of  Proteids. — A  second  group  of  foods 
formed  by  plants  are  called  proteids.     These  differ  from  the  car- 
bohydrates in  that  they  contain  nitrogen  in  addition  to  carbon, 
hydrogen  and  oxygen.     They  are  more  complex  compounds  than 
the  carbohydrates  and  may  contain  sulphur  and  phosphorus  in 
addition  to  the  four  elements  mentioned  above.     Less  is  known 
about  their  formation  than  of  the  carbohydrates.     It  is  probable 
that  they  are  largely  formed  in  the  leaves  and  by  a  process  simi- 
lar to  that  of  photosynthesis.     The  sulphates,  phosphates,  and 
nitrates  absorbed  from  the  soil  are  decomposed  and  the  elements 
of  nitrogen,  sulphur  or  phosphorus  are  united  to  simple  carbon 
compounds  and  complex  proteids  are  the  result.     Light  probably 
does  not  co-operate  directly  in  this  construction  although  it 
may  do  so  indirectly.     These  foods  are  formed  in  much  smaller 
quantities  than  the  carbohydrates  but  they  are  of  the  greatest 
importance  in  the  nourishment  of  the  plant.     This  is  especially 
true  as  regards  the  living  substance,  protoplasm,  which  resembles 
somewhat  in  composition  some  of  the  more  complex  proteids. 

9.  The  Distribution  of  the  Foods. — Let  us  now  consider  what 
becomes  of  these  foods.     A  small  part  is  consumed  on  the  spot 
by  the  manufacturing  cells  themselves,  a  larger  portion  is  trans- 
ported  through  the  vascular  bundles  to  all  the  living  and  grow- 
ing cells  of  the  plant  body,  but  as  the  plant  approaches  the  com- 
pletion of  its  annual  growth  a  larger  and  larger  part  of  the  food 


NATURE  OF  PLANTS  17 

is  transferred  to  special  parts  of  the  plant,  such  as  buds,  roots, 
seeds.  Here  it  is  stored  as  a  reserve  food  to  meet  the  needs  of 
the  plant  at  such  times  as  it  is  not  able  to  manufacture  food. 
This  transfer  of  foods  is  slow  and  consequently  the  rapidly  con- 
structed sugars  gradually  accumulate  in  the  chlorenchyma  during 
the  day.  This  would  result  in  the  saturation  of  the  cells  with 
sugar  and  so  stop  the  work  of  photosynthesis  were  it  not  for  the 
fact  that  the  chloroplasts  quickly  change  the  sugar  to  insoluble 
starch,  thus  leaving  the  cells  free  to  receive  more  sugar.  If 
chloroplasts  of  well-sunned,  starch-forming  leaves  are  examined 
in  the  afternoon,  they  will  be  found  to  contain  minute  glistening 
bodies,  the  starch  grains  (Fig.  9,  s).  The  chloroplasts  have  the 
power  to  absorb  sugar  and  secrete  starch.  The  construction  of 


FIG.  9.  Greatly  enlarged  chloroplasts  from  leaf  of  moss:  a,  plastid  with 
three  starch  grains,  s;  b,  plastid  elongating  preparatory  to  division;  c,  d, 
later  stages  in  division. 

sugars  and  their  transformation  into  starch  is  effected  with  sur- 
prising rapidity.  If  some  plants  of  pond  scum  are  placed  in  the 
dark  for  twenty-four  hours  so  that  all  sugars  and  starches  may 
be  consumed,  starch  will  re-appear  in  the  cells  in  these  plants  in 
from  three  to  five  minutes  after  their  return  to  the  light.  Con- 
sider the  absorptions,  decompositions,  recompositions,  and  trans- 
formations hat  have  been  effected  in  this  short  time.  Is  it 
surprising  that  the  exact  nature  of  the  changes  effected  in  the 
manufacture  of  foods  is  not  known?  It  is  evident  as  a  result  of 
the  rapid  formation  of  sugar  that  starch  must  accumulate  in 
the  plastids  during  the  day.  At  night  sugar  is  no  longer  formed 
but  the  transfer  of  food  continues  as  in  the  day.  The  insoluble 
starch  is  now  dissolved  by  means  of  a  ferment  or  enzyme  into 


18 


STORAGE  OF  FOODS 


sugar  so  that  by  morning  the  cells  are  quite  emptied  of  the  sugar 
and  ready  for  another  day's  work.  This  accumulation  and  trans- 
fer of  foods  can  easily  be  demonstrated  by  cutting  off  in  the 
afternoon  leaves  from  clover,  bean  or  other  starch-forming  plant 
and  placing  them  in  alcohol.  When  blanched  and  tested  with 
iodine  the  blue  or  blue-black  color  shows  that  they  have  accumu- 
lated a  large  amount  of  starch.  Test  in  the  same  way  leaves 
taken  from  these  plants  early  in  the  morning.  No  starch  re- 
action will  be  seen  because  the  cells  have  been  freed  from  their 
starch  during  the  night.  The  formation  of  starch  is  often  used 
as  a  test  for  photosynthesis,  but  it  must  be  borne  in  mind  that 
photosynthesis  first  results  in  the  formation  of  various  sugars 
and  that  starch  only  appears  when  the  sugars  have  reached  a 
certain  percentage  in  the  cells  and  indeed  that  some  plants  do 
not  form  starch  at  all.  So  when  starch  appears  in  a  leaf  it  is 
simply  the  measure  of  the  excess  of  carbohydrate  manufacture 
over  transportation.  Furthermore  starch  will  form  in  the  cells 
in  the  dark  if  a  plant  is  supplied  with  an  excess  of  carbohydrate. 
10.  The  Storage  of  Foods. — It  has  been  stated  that  the  foods 
not  required  for  the  nourishment  and  growth  of  the  plant  are 
stored  in  the  region  of  buds,  the  growing  portions  of  stems, 


FIG.  10.     Storage  food:  A,  starch  grains  in  cell  of  potato — n,  nucleus. 
B,  cells  of  bean,  the  smaller  particles  being  proteid  grains. 

in  bulbs,  in  roots,  in  seeds,  and  in  fruits.  These  are  the  so- 
called  storage  foods  and  they  remain  in  these  regions  for  con- 
siderable periods  or  until  the  conditions  are  favorable  for  a  re- 
newal of  growth.  They  generally  assume  much  more  definite 


NATURE   OF   PLANTS 


forms  than  the  products  constructed  in  the  chlorenchyma  which 
were  designed  for  immediate  use  or  translocation  to  other  parts. 
If  we  examine  a  potato  the  cells  will  seem  to  be  filled  with 
very  regularly  constructed  starch  grains  (Fig.  10,  A).  In  the 
pea  seed  and  in  wheat  and  other  grains  proteid  granules  are 
associated  with  the  starch  grains  (Fig.  10,  B).  These  starch 
grains  are  formed  by  rather  colorless  plastids  called  leucoplasts 
(Fig.  3,  E)  that  closely  resemble  the  chloroplasts.  The  starch 
grain  first  appears  in  the  leucoplast  as  a  minute  point.  Its  strati- 
fied structure  is  due  to  the  successive  deposits  of  starch  by  the 
leucoplasts.  If  the  grain  is  built  up  equally  on  all  sides  by  the 
leucoplasts  it  will  possess  regular  and  concentric  strata  as  in  the 
bean  (Fig.  n,  A).  If  the  bulk  of  the  leucoplast  lies  on  one 
side  of  the  grain,  this  side  of  the  grain  will  receive  more  material 
and  consequently  the  starch  grain  will  become  more  or  less  one- 


FIG.  ii.  FIG.  12. 

FIG.  II.  Starch  grains:  A,  from  bean.  B,  from  potato.  C,  compound 
.grain  from  potato. 

FIG.  12.  Section  of  the  outer  portion  of  a  grain  of  wheat:  p,  cells  containing 
proteid  grains.  The  larger  cells  are  filled  with  starch. — After  Strasburger. 

sided  as  in  the  potato  (Fig.  n,  B}.  Frequently  two  or  more 
grains  originate  in  one  leucoplast  and  compound  grains  result 
(Fig.  ii,  C).  Proteids  and  other  foods  are  likewise  transported 
in  solution  to  the  storage  organs  where  they  may  or  may  not 
be  deposited  in  solid  form.  This  is  usually  effected  directly  by 
the  protoplasm  of  the  cell  without  the  aid  of  any  plastid.  Ani- 
mals have  learned  to  use  the  foods  stored  in  these  organs  just 
as  does  the  plant.  It  is  interesting,  however  to  note  that  we 


20  NATURE   OF   ENZYMES 

cannot  use  them  as  economically  as  the  plant  does.  In  the 
wheat,  for  example,  the  proteid  material  is  largely  confined  to 
the  outer  portion  of  the  grain  (Fig.  12).  This  layer  is  of  a 
darker  color  and  is  excluded  in  the  milling  process,  leaving 
the  flour  very  white.  So  the  most  nutritious  part  of  the  wheat 
does  not  appear  in  white  bread,  but  the  proteid  material  is  digest- 
ible to  only  a  slight  degree.  The  removed  portions,  however, 
do  contain  substances,  as  vitamen,  that  are  beneficial  to  health. 
These  reserve  foods  are  made  available  to  the  plant  in 
the  same  way  as  to  the  animal.  They  are  first  put  into 
solutions  by  ferments  or  enzymes.  Ptyalin  in  the  saliva  of 
animals  and*  diastase  in  the  plant  are  common  examples 
of  enzymes  which  change  starch  into  sugar.  So  there  are 
enzymes  which  transform  each  reserve  food  and  render  it  ca- 
pable of  transport  and  incorporation  into  the  substance  of  the 
plant  body.  These  enzymes  are  formed  by  the  living  substance 
of  certain  cells  and  resemble  in  many  ways  the  living  matter 
itself.  They  are  most  extraordinary  in  their  action  and  behavior. 
The  decomposition  of  the  substances  upon  which  they  react  is 
effected  by  the  mere  presence  of  the  ferment — at  least  there  is 
no  permanent  union  between  ferment  and  the  substance.  Conse- 
quently a  very  small  amount  of  a  ferment  may  effect  a  decom- 
position of  an  almost  unlimited  amount  of  a  given  substance. 
More  commonly  water  enters  into  the  composition  of  the  sub- 
stance as  a  result  of  the  presence  of  the  enzyme.  As  a  result  its 
stability  is  upset  and  it  breaks  down  into  simpler  compounds. 
This  decomposition  due  to  the  incorporation  of  water  is  termed, 
for  this  reason  hydrolysis.  We  say  that  diastase  hydrolyzes 
starch  into  simpler  compounds,  as  maltose  and  dextrines. 
Some  enzymes  appear  to  effect  decompositions  without  the  co- 
operation of  other  substances.  Still  more  remarkable  is  the 
action  of  these  ferments  in  that  some  of  them  appear  to  have  a 
reverse  action,  i.  e.,  the  power  of  building  up  again  complex 
material  from  the  parts  into  which  they  have  separated  it.  In 
many  ways  enzymes  behave  like  living  matter,  being  quickened 
in  their  activities  by  suitable  temperatures,  the  concentration 
of  the  fluid  or  by  the  presence  of  certain  acids  and  alkalies;  or 


NATURE   OF   PLANTS  21 

retarded  by  solutions  containing  copper  or  mercury  which  also 
poison  living  matter.  The  distribution  of  foods  made  possible 
by  enzymes  has  been  compared  to  the  currency  of  a  country; 
the  storage  foods  are  the  bank  reserve  while  the  sugars  and  other 
solutions  are  the  circulating  currency. 

11.  Significance  of  the  Leaf  Structure. — We  now  begin  to 
understand  the  significance  of  the  structure  and  form  of  the  leaf. 
The  broad  blade  is  a  device  to  catch  the  energy  of  the  sunlight. 
The  stomata  afford  access  of  CO2  and  escape  of  oxygen.     The 
intercellular  spaces  in  the  spongy  mesophyll  all   lead   to  the 
stomata,  thus  bringing  about  a  quick  distribution  of  the  gases 
and  they  also  increase  the  area  for  the  absorption  of  CO2  and 
excretion  of  oxygen  to  such  an  extent  that  the  internal  surface 
of  the  leaf  exceeds  the  external  many  times.     It  will  be  noticed 
that  the  cells  of  the  spongy  mesophyll  vary  in  character.     This  is 
because  of  the  different  functions  that  they  perform.     The  col- 
lecting cells  (Fig.  5,  col)  are  closely  applied  to  the  palisade  cells 
and  collect  the  sugars  and  other  foods  from  them  while  the  elon- 
gated conducting  cells  transport  this  food  to  the  cells  of  the 
vascular  bundles. 

12.  The  Second  Function  of  the  Leaf. — Respiration  or  breath- 
ing is  a  second  function  of  leaves.     While  work  of  this  kind  is 
performed  by  all  living  cells  it  may  be  considered  at  this  point 
because  in  the  leaves  the  nature  of  the  work  is  very  well  illus- 
trated.    The  work  of  respiration  is  usually  associated  with  the 
absorption  of  oxygen  and  the  giving  off  of  CO2,     Animals  and 
plants  are  constantly  taking  in  oxygen  and  giving  off  CO2.     This 
interchange  of  gases  is  the  reverse  of  that  occurring  in  photo- 
synthesis.    Furthermore  respiration  goes  on  all  the  time  when 
conditions  are  suitable  for  life  while  photosynthesis  only  is  pos- 
sible in  the  light.     Let  us  consider  the  significance  of  this  inter- 
change of  gases.     Why  do  animals  and  plants  breathe?     The 
construction  of  the  foods  to  which  attention  has  been  called  may 
well  be  termed  the  storing  up  of  the  energy  of  the  sunlight. 
When  the  chlorophyll  absorbs  certain  rays  of  light  it  does  not 
destroy  any  of  the  energy  in  those  rays.     The  energy  is  changed 
to  another  form  and  it  still  exists  in  the  sugars  and  other  com- 


22  NATURE   OF   RESPIRATION 

pounds  that  are  built  up  through  its  power.  The  energy  is 
locked  up,  so  to  speak,  in  the  foods  and  finally  it  is  lodged  in  the 
compounds  that  are  constructed  from  the  foods,  i.  e.,  in  the 
living  substance  and  in  the  tissues  of  the  plant.  If  now  any  of 
these  compounds  are  decomposed  into  their  constituent  parts 
then  the  energy  will  be  set  free  unimpaired.  Oxygen  is  the 
principal  agent  in  carrying  on  this  decomposition.  It  has  a 
great  attraction  for  nearly  all  the  elements  occurring  in  the  plant 
and  so  it  is  able  to  detach  one  element  from  another  and  so 
effect  decomposition  and  the  liberation  of  the  locked  up  energy. 
You  are  not  to  think  of  the  oxygen  entering  directly  into  combi- 
nation with  the  foods  or  with  the  substances  formed  from  the 
foods  but  rather  with  the  decomposition  products  that  are 
brought  about  by  the  living  matter.  It  is  probable  that  respira- 
tion starts  in  the  living  substance.  Some  agent,  other  than 
oxygen  and  possibly  enzymic  in  nature,  inaugurates  these 
decompositions  and  then  oxygen  comes  in  at  some  stage  in  this 
breaking  down  process  and  makes  possible  a  further  reduction. 
As  evidence  that  the  oxygen  does  not  combine  directly  with  the 
more  complex  substances  mention  may  be  made  of  the  fact  that 
the  absorption  of  oxygen  is  quite  independent  of  the  amount  of 
oxygen  present.  The  plant  respires  at  the  same  rate  in  an 
atmosphere  of  pure  oxygen  as  in  an  atmosphere  containing  one 
twentieth  part  of  the  oxygen  in  the  air.  Furthermore  the 
amount  of  oxygen  absorbed  is  independent  of  the  amount  of 
foods  stored  in  the  plant.  So  respiration  is  not  a  simple  and 
direct  process  like  combustion  where  the  percentage  of  oxygen 
and  the  amount  of  material  present  determine  the  reaction. 

In  the  majority  of  cases  oxygen  causes  a  continuation  of  the 
decomposition  processes  until  such  simple  substances  as  COj 
and  HO  compounds,  such  as  water,  are  formed — in  a  word 
until  the  very  substances  utilized  in  photosynthesis  are  formed. 
Some  of  these  simple  products,  such  as  H2O  and  CO2,  escape 
from  the  stomata  as  gas  and  water  vapor.  Accordingly  respira- 
tion is  usually  indicated  by  the  absorption  of  oxygen  and  the  giv- 
ing off  of  CO2  and  H2O.  Some  plants,  however,  have  sufficient 
oxygen  in  their  tissues  to  enable  them  to  breathe  for  considerable 


NATURE  OF  PLANTS  23 

periods  without  absorbing  oxygen  from  the  air  and  on  the  other 
hand  in  some  plants  CO2  is  not  always  given  off  because  the 
decomposition  is  not  carried  so  far  as  to  result  in  the  formation 
of  so  simple  a  product.  We  eat  in  order  to  gain  possession  of 
the  energy  locked  up  in  the  foods.  We  breathe  in  order  that  the 
oxygen  may  assist  in  the  decomposition  of  these  compounds  and 
set  free  this  energy  which  gives  us  power  to  work  and  move 
and  keep  our  bodies  warm.  The  plant  lives  in  the  same  way. 
It  only  differs  from  the  animal  in  that  it  has  the  added  power  to 
build  up  the  complex  food  compounds  from  crude  material. 
By  the  decomposition  of  the  products  formed  from  these  foods  it 
gains  energy  to  grow  and  carry  on  its  other  vital  functions. 
The  work  of  respiration  is  carried  on  more  economically  by  green 
plants  than  by  animals  since  in  the  animal  the  CO2  escapes  in  the 
breath  as  a  waste  product,  while  the  plant  uses  this  CO2  during 
the  day  time  for  the  construction  of  foods.  Consequently  the 
escape  of  CO2  can  only  be  observed  during  the  night  and  cannot  be 
detected  in  the  light  unless  an  examination  of  very  rapidly  grow- 
ing organs  be  made.  If  a  jar  be  nearly  filled  with  opening  buds 
of  dandelions  or  rapidly  growing  shoots  and  then  closed  air  tight, 
sufficient  CO2  will  be  respired  in  a  few  hours  to  extinguish  a  light 
that  is  lowered  into  the  jar.  In  such  instances  as  these  very 
rapid  respiration  is  necessary  to  furnish  the  required  energy  for 
growth. and  the  volumes  of  CO2  expired  exceed  many  times  the 
volume  of  CO2  utilized  in  photosynthesis.  A  handful  of  ger- 
minating peas  or  beans  placed  in  a  closed  jar  for  a  few  hours  better 
illustrates  the  giving  off  of  CO2  because  here  there  is  no  green 
tissue  to  absorb  any  of  the  CO2.  Plants  are  often  considered 
unhealthful  in  sleeping  rooms  at  night  because  of  their  exhalation 
of  CO2.  It  is  well  to  remember  that  the  amount  of  CO2  expired 
by  a'plant  is  small  and  that  a  gas  jet  would  furnish  more  CO2  to 
the  air  than  a  window  full  of  plants.  A  square  meter  of  leaf 
surface  gives  off  about  .12  gm.  (60  c.c.)  of  carbon  dioxide  per 
hour  at  20°  C. 

We  are  now  in  a  position  to  understand  the  importance  of 
photosynthesis  and  respiration.  Photosynthesis  keeps  the  air 
pure  for  breathing,  decomposes  the  simple  inorganic  compounds 


24  AMOUNT   OF  TRANSPIRATION 

and  recombines  them  into  foods  which  represent  a  certain  amount 
of  stored-up  energy.  Respiration  breaks  down  the  products 
formed  from  these  foods  and  liberates  the  energy  necessary  for 
the  growth  and  activities  of  the  plant. 

13.  The  Third  Function  of  the  Leaf,  Transpiration. — This 
function  refers  to  the  giving  off  of  water  by  the  plant.  While 
other  parts  of  the  plant  assist  in  transpiration,  the  leaf  is  the 
principal  organ  upon  which  this  very  considerable  work  devolves. 
Water  is  given  off  from  the  plant  as  a  vapor  and  for  this  reason 
transpiration  is  a  more  familiar  phenomenon  than  photosynthesis 
and  respiration,  where  we  are  dealing  with  an  interchange  of 
invisible  gases.  We  see  the  vapor  from  plants  growing  in  a 
window  precipitated  on  the  cool  window  panes  in  the  form  of 
drops.  On  hot  summer  days  the  leaves  of  plants  droop.  This 
is  because  they  have  transpired  so  much  water  that  their  cells 
are  no  longer  distended  by  the  water,  consequently  the  cells  shrink 
and  the  leaves  contract  or  wilt.  When  plants  are  covered  by  a 
bell  jar  or  placed  in  a  tight  glass  jar  the  water  given  off  soon 
saturates  the  air  and  collects  in  drops  on  the  sides  of  the  jar. 
The  amount  of  water  transpired  by  a  plant  is  surprisingly  large 
and  it  is  probably  safe  to  state  that  usually  it  amounts  daily 
during  the  hot  summer  months  to  more  than  the  plant's  weight. 
An  oak  with  seven  hundred  thousand  leaves  was  estimated  by 
Ward  to  transpire  from  June  to  October  244,695  pounds  of  water, 
A  birch  with  200,000  leaves  transpired  700  to  900  gallons  on  hot 
summer  days.  This  means  that  an  acre  of  such  trees  would  give 
off  in  the  course  of  the  season  3,168,000  pounds  of  water.  From 
careful  measurements  of  the  amount  of  water  given  off  by  grass 
plants  it  has  been  calculated  that  six  and  one-half  tons  per 
acre  may  be  transpired  daily  during  .the  summer.  It  is  estimated 
that  from  200  to  500  Ibs.  of  water  is  transpired  in  the  construction 
of  one  pound  of  dry  substance  and  that  a  square  meter  of  leaf 
surface  evaporates  about  50  gms.  per  hour. 

The  question  naturally  arises  why  are  such  large  volumes  of 
water  transpired?  This  depends  in  part  upon  the  size  of  the  leaf 
and  external  conditions,  the  size  of  the  leaf  being  determined  by 
the  relation  of  transpiration  to  photosynthesis.  Conditions  mak- 


NATURE  OF  PLANTS  25 

ing  possible  vigorous  photosyntheses  would  result  in  extensive 
growth  and  leaf  development.  An  extensive  leaf  area,  however, 
may  not  necessitate  a  heavy  transpiration;  the  amount  of  water 
given  off  will  depend  upon  external  conditions.  In  warm  moist 
climates  conditions  are  favorable  for  photosynthesis  but  not  for 
transpiration,  which  is  reduced  in  amount  by  the  moist  air. 
So  growth  is  vigorous,  the  largest  leaves  are  developed  and 
transpiration  is  feeble.  In  warm  but  dry  climates,  on  the  other 
hand,  while  conditions  are  favorable  for  photosynthesis  the  dry 
air  promotes  excessive  transpiration.  This  interferes  with 
photosynthesis,  consequently  very  small  leaves  develop  and  the 
volume  of  water  transpired  is  correspondingly  reduced.  Under 
conditions  of  moderate  temperature  and  moisture  as  in  most 
temperate  climates,  the  leaf  assumes  a  size  intermediate  between 
these  two  extremes  because  these  conditions  favor  vigorous 
photosynthesis  and  the  resulting  growth  is  not  materially  checked 
by  transpiration.  So  we  see  that  the  extensive  transpiration  in 
temperate  regions  is  in  part  the  result  of  the  large  leaf  develop- 
ment and  that  this  leaf  area  is  dependent  upon  a  vigorous  photo- 
synthesis. 

Only  a  small  part  of  the  water  absorbed  by  the  plant  is  re- 
quired to  furnish  the  necessary  mineral  substances  and  the  water 
utilized  in  its  growth.  The  surplus  is  forced  out  of  the  cells  and 
finds  its  way  in  the  form  of  vapor  along  the  intercellular  spaces 
in  the  spongy  mesophyll  through  the  stomata  to  the  air.  The 
transpiration  of  these  large  volumes  of  water  must  be  of  impor- 
tance in  keeping  down  the  temperature  of  the  plant  during  the 
burning  summer  heat.  Transpiration  is  often  compared  'to  the 
evaporation  of  water  from  a  dish.  While  it  is  controlled  to  a 
limited  extent  in  the  same  manner  as  evaporation,  it  should  be 
borne  in  mind  that  the  giving  off  of  water  is  intimately  associated 
with  the  vital  activities  of  the  cells  and  that  the  loss  of  water  is 
to  a  degree  under  the  control  of  the  plant.  The  cuticle,  which  is 
practically  impervious  to  water  and  gases,  extends  as  a  coat  over 
all  parts  of  the  plant  body,  and  materially  assists  in  controlling 
the  amount  of  transpiration.  The  thickness  of  this  protective 
coating  depends  upon  external  conditions,  being  developed  in 
3 


26  THE  STOMATA 

proportion  to  the  ratio  of  transpiration  to  absorption.  So  we 
find  that  it  becomes  thicker  as  the  volume  of  water  given  off 
comes  nearer  and  nearer  to  the  volume  absorbed.  In  this  way  it 
serves  as  a  very  effective  check  in  preventing  a  fatal  loss  of  water. 
As  a  result  of  this  water-proof  coat  the  vapor  can  only  escape  from 
the  leaf  through  the  stomata.  These  minute  openings  are  one 
of  the  most  interesting  features  of  the  leaf.  When  there  is  an 
adequate  supply  of  water  the  elliptical  guard  cells  are  drawn  apart 
and  give  free  exit  to  vapors  and  gases.  However  in  cases  of 
drought  when  a  continued  loss  of  water  would  prove  harmful  to 
the  plant,  or  when  an  interchange  of  gases  is  no  longer  required 
then  the  guard  cells  close  and  so  offer  such  a  barrier  against 
further  loss  of  water  that  only  the  severest  conditions,  such  as 
prolonged  heat  and  drought,  can  overcome.  It  has  been  claimed 
that  the  stomata  do  not  regulate  transpiration  because  it  has 
been  observed  in  some  cases  that  the  stomata  are  not  closed  when 
wilting  begins  or  fully  opened  at  the  time  of  maximum  transpira- 
tion. Further  observation  is  required  to  settle  this  question  and 
for  the  present  we  must  believe  that  so  elaborate  a  mechanism 
as  the  guard  cells  is  of  significance.  The  stomata  in  the  majority 
of  cases  range  in  size  from  .0002  sq.  mm.  to  .0008  sq.  mm.  so  that 
in  comparison  a  needle  prick  would  appear  as  a  huge  hole,  but 
they  are  so  numerous,  40  to  300  to  the  sq.  mm.,  as  to  comprise 
about  I  per  cent,  to  3  per  cent,  of  the  area  of  the  leaf  surface. 
It  might  be  questioned  if  this  extent  of  opening  is  sufficient  to 
permit  the  entrance  and  exit  of  the  large  volumes  of  gases  and 
water  handled  by  the  plant.  It  has  been  shown  that  a  mem- 
brane pierced  by  sufficiently  small  openings  which  are  separated 
from  one  another  by  a  distance  of  at  least  ten  times  their  diam- 
eter, permits  diffusion  of  gases  as  readily  as  though  there  were 
no  membrane  at  all.  The  structure  of  the  epidermis  is  an 
admirable  example  of  such  a  perforated  membrane  and  it  is  so 
perfectly  adjusted  to  the  work  in  hand  that  it  could  accomplish 
much  more  than  the  necessities  of  the  plant  demand. 

14.  Noteworthy  Features  of  Leaves. — Now  that  some  idea  has 
been  gained  of  the  nature  and  extent  of  the  work  performed  by 
the  leaves  we  are  prepared  to  comprehend  the  meaning  of  the 


NATURE   OF   PLANTS  27 

large  extent  of  the  leaf  surface  and  of  the  arrangement,  structure 
and  modification  of  the  leaves.  The  broad  blades  are  devices  for 
gathering  or  absorbing  as  much  of  the  sunlight  as  possible  and 
they  increase  the  surface  of  the  plant  many  hundreds  of  times. 
Contrast  the  extent  of  surface  of  a  tree  in  full  foliage  with  the 
bare  branches  of  the  winter.  A  mature  maple  develops  annually 
from  one  to  two  thousand  square  yards  of  leaf  surface. 

One  of  the  most  noteworthy  features  about  the  leaves  is  their 
arrangement  or  "hang"  on  the  branches.  In  some  plants  they 
are  arranged  in  two  vertical  rows,  in  other  instances  three,  four, 
five,  eight  or  more  rows.  By  fastening  a  thread  to  a  leaf  and 
winding  it  about  the  stem  so  as  to  touch  the  petiole  of  each  suc- 
ceeding leaf  the  arrangement  of  the  leaves  becomes  more  obvious. 
Some  plants  have  their  leaves  opposite  in  two  rows  or  ranks, 
others  opposite  in  four  ranks,  each  succeeding  set  of  leaves  being 
at  right  angles  to  the  lower  set.  This  latter  arrangement  is  called 
decussate  (Fig.  13).  More  commonly  the  leaves  are  spirally 
arranged  and  the  thread  passed  once,  twice,  thrice,  five,  eight, 
etc.,  times  around  the  stem  before  a  leaf  is  reached  that  is  exactly 
over  the  one  from  which  we  started  (Fig.  14).  The  number  of 
leaves  passed  before  reaching  one  that  stands  over  the  first  leaf 
indicates  the  number  of  rows  or  ranks  of  leaves  on  the  stem.  This 
variation  in  the  arrangement  of  the  leaves  is  simply  a  device  to 
bring  the  leaves  into  the  light  and  prevent  the  shading  of  one  leaf 
by  another.  In  opposite  two  ranked  leaves  each  succeeding  leaf 
is  over  the  one  below  it  but  the  cutting  off  of  the  light  is  pre- 
vented by  lengthening  the  internodes  and  -thus  separating  the 
leaves.  The  decussate  arrangement  of  the  leaves  is  a  more  eco- 
nomical arrangement  because  more  leaves  can  be  developed  on 
a  given  length  of  stem  without  danger  of  shading.  The  spiral 
distribution  of  leaves  is  still  a  better  device  since  each  succeeding 
leaf  is  placed  a  little  above  and  to  one  side  of  the  next  lower  leaf. 
By  this  means  the  maximum  of  leaf  surface  can  be  exposed  to 
the  light  in  a  given  length  of  stem.  No  one  can  look  at  the  leaves 
of  the  maples,  dogwoods,  ailanthus,  creeping  and  climbing  vines, 
etc.,  and  not  be  impressed  with  the  fact  that  the  leaves  are 
arranged  so  as  to  catch  the  most  favorable  light  without  shading 


28 


LEAF  ARRANGEMENT 


or  interfering  with  one  another.     Furthermore  the  arrangement 
is  such  as  to  occupy  practically  all  the  space  about  the  stem  that 


FIG.  13.  FIG.  14. 

FIG.  13.     Branch  of  Forsythia,  leaves  decussate,  in  four  rows. 
FIG.  14.     Branch  of  poplar  with  leaves  spirally  arranged  in  five  rows. 

receives  light.  This  becomes  very  noticeable  if  we  look  directly 
down  upon  a  shoot  or  better  if  we  stand  under  a  tree  and  look 
up  into  the  branches.  It  will  be  seen  that  very  little  direct  sun- 
light finds  its  way  through  the  branch,  so  nicely  are  the  leaves 
adjusted  to  each  other.  Some  plants  like  the  hickories,  catalpa, 
etc.,  occupy  all  the  available  space  with  a  few  large  leaves  at  the 
tips  of  the  branches.  In  other  cases,  as  in  the  willows,  some 
lilies,  etc.,  the  same  result  is  accomplished  by  many  small  leaves 
that  can  be  arranged  along  the  stem  for  considerable  distances 
without  shading.  The  nicety  of  leaf  arrangements  is  especially 
noticeable  in  many  horizontal  and  creeping  stems.  In  such  cases 
the  leaves  can  only  be  exposed  on  the  sides  of  the  stem  and  con- 
sequently the  stem  may  become  twisted  or  more  usually  the 


NATURE   OF   PLANTS 


29 


petioles  are  variously  elongated  and  curved  in  order  to  bring  the 
leaves  into  proper  relation  to  the  light  (Fig.  15).     Notice  the 


FIG.  15.  Horizontal  branch  of  Forsythia  with  leaves  in  two  rows  owing  to 
the  alternate  twisting  of  each  internode,  assisted  by  the  curving  of  the  petioles. 
Compare  Fig.  13. 

change  in  leaf  arrangement  in  horizontal  and  erect  stems  of  maple. 
In  an  example  like  the  maple  if  the  leaves  could  not  change  their 
position  they  would  all  be  standing  edgewise  to  the  light  on  hori- 
zontal branches  and  therefore  receive  little  of  it.  They  not  only 
place  their  blades  at  right  angles  to  the  light  but  owing  to  the 
greater  elongation  of  each  succeeding  petiole  from  the  apex 
toward  the  base  of  the  stem,  all  the  leaves  are  arranged  one  be- 
yond another  so  as  to  overlap  very  little.  This  same  device  is 
noticed  in  many  plants  that  produce  their  leaves  close  to  the 
ground  in  rosettes,  as  mullein,  wood  betony,  plantain,  etc.  (Fig. 


FIG.  1 6.     Leaf  rosette  of  evening  primrose.     Each  leaf  blade  has  a  longer 
petiole  than  the  one  above  it,  so  that  the  leaves  are  not  shaded  by  one  another. 

1 6).     Compare  the  erect  and  horizontal  branches  of  a  variety  of 
plants  noting  by  what  devices  the  leaves  are  brought  into  the 


30  FORM   OF  LEAVES 

light.  Especially  instructive  are  the  leaf  arrangements  of  plants 
growing  in  windows  or  creeping  over  trellises,  etc.,  where  compli- 
cated twisting  and  elongation  of  stems  and  petioles  are  necessary 
to  adjust  the  leaves  to  the  one-sided  illumination.  The  angular 
leaves  of  some  begonias  furnish  excellent  illustrations  of  this. 
Perhaps  so  many  plants  bear  angular  leaves  because  they  can  be 
better  adjusted  and  fitted  together  without  loss  of  space  than 
would  be  the  case  in  rounded  leaves,  and  smaller  leaves  can  also 
be  more  advantageously  introduced  between  the  larger  ones 
(Fig.  17).  The  lobing  and  branching  of  leaves  has  become  a 
characteristic  of  many  plants  because  such  variations  permit  the 


FIG.  17.     Leaf  of  Aralia.     Note  the  angular  shape  of  the  leaflets  and  the 
smaller  ones  filling  the  space  between  the  larger  ones. 

illumination  of  a  larger  leaf  surface.  This  is  particularly  notice- 
able in  our  oaks  where  the  outer  leaves  are  often  deeply  lobed, 
thus  permitting  considerable  light  to  pass  through  to  the  under- 
lying leaves.  Observe  also  that  these  lower  leaves  are  larger  and 
less  lobed,  thus  catching  as  much  as  possible  of  this  rather  feeble 
light  (Fig.  1 8).  In  many  plants  the  lobing  extends  quite  to  the 
middle  of  the  leaf  and  the  lobes  are  often  attached  to  the  midvein 
or  midrib  by  a  petiole.  In  this  latter  case  the  leaf  is  said  to  be 
compound  (Fig.  19).  All  such  modifications  permit  the  develop- 
ment of  numerous  leaves  upon  the  branches  without  the  danger 
of  shading.  If  there  is  still  any  doubt  as  to  the  perfection  of  this 
light-catching  arrangement  of  the  leaves,  try  to  substitute  the 
somewhat  similar  leaves  of  two  different  trees  as  the  birch  and  elm 


NATURE  OF  PLANTS  31 

or  the  water  beech  (Carpinus)  and  the  beech,  noting  how  the 
leaves  of  one  fit  when  placed  upon  the  branch  of  the  other.  This 
subject  of  leaf  form  and  arrangement  may  be  summed  up  by  the 
statement  that  these  features  are  devices  for  exposing  the 
maximum  leaf  surface  without  one  leaf  interfering  with  another. 
This  is  in  accord  with  the  fact  that  light  is  practically  of  no  value 
in  photosynthesis  after  passing  through  two  leaves. 

Another  interesting  feature  about  the  leaf  is  its  relation  to  the 


i/B 

FIG.  18.  FIG.  19. 

FIG.  1 8.     Leaves  of  red  oak:  A,  sunned  leaf.     B,  shaded  leaf. 
FIG.  19.     Compound  leaves:  A,  red  ash.     B,  horse  chestnut. 

intensities  of  light.  Some  plants  demand  the  full  intensities  of 
sunlight  while  others  can  tolerate  only  a  small  fraction  of  it. 
Some  lichens  will  grow  in  a  light  only  1/156  part  of  the  full  in- 
tensity while  many  plants,  as  the  grasses,  will  endure  the  strongest 
illumination.  Beech,  maple  and  spruce  are  tolerant  of  shade  and 
oak,  hickory  and  chestnut  are  intolerant.  This  relation  of  plants 
to  light  is  an  important  factor  in  controlling  their  associations 
and  we  frequently  speak  of  plants  as  sun  loving,  partially  so, 
shade,  and  deep  shade  plants. 

The  coloration  of  leaves  is  often  of  significance.     In  addition 
to  the  green  color,  of  which  we  now  have  some  understanding, 


32  LEAF  COLORATION 

a  variety  of  other  colors  often  appears.  One  of  the  most  common 
is  red.  Young  shoots  of  the  rose,  grape  vine  and  so  much  of  the 
early  spring  vegetation  are  tinged  with  red.  So  in  the  fall  our 
vegetation  takes  on  a  greater  wealth  of  coloration  than  is  seen 
in  any  other  land.  And  finally  there  are  leaves  variegated  with 
yellow,  white  and  red,  colored  fruits  and  other  organs,  and  the 
endless  hues  of  the  flowers.  These  colors  are  caused  by  pigments 
or  by  chromoplasts  (p.  2)  that  are  developed  in  certain  cells  and 
which  either  transmit  or  reflect  to  the  eye  their  particular  colors- 
The  white  blotches  of  variegated  leaves  are  caused  by  the  absence 
of  chlorophyll,  thus  allowing  the  light  to  pass  practically  un- 
changed, or  in  some  cases  these  areas  are  characterized  by  large 
intercellular  spaces  filled  with  air.  Such  a  structure  would  tend 
to  reflect  the  light  and  so  contribute  to  the  white  appearance  of 
the  area  (p.  39).  In  some  cases  these  colors  may  be  of  service  to 
the  plant.  The  red  probably  functions  as  a  screen  to  young  or- 
gans, shielding  them  from  the  intense  light  that  would  otherwise 
decompose  the  forming  chlorophyll  and  otherwise  interfere  with 
the  vital  processes.  Red  is  also  a  strong  absorber  of  the  heat 
rays  in  light,  and  in  some  cases  it  may  be  of  service  in  ensuring  a 
higher  temperature  in  the  organs  and  so  expedite  their  work. 
Many  plants,  some  tradescantias,  hawkweeds  and  spatter  dock, 
have  in  their  mature  leaves  layers  of  cells,  frequently  the  lower 
epidermis,  filled  with  a  red  pigment.  So  many  evergreens  assume 
a  brown  or  reddish  hue  in  the  winter  owing  to  the  formation  of  a 
reddish  pigment.  However,  too  much  emphasis  should  not  be 
given  to  this  matter  of  coloration.  The  colors  of  flowers,  to  be 
sure,  are  of  service  in  guiding  insects  when  close  at  hand,  to  the 
proper  approach  to  the  flower,  and  there  is  probably  a  significance 
in  the  coloration  of  the  algae  (p.  173)  but  in  general  it  is  not 
possible  at  present  to  offer  an  explanation  for  the  variety  of  colors 
that  characterize  so  many  leaves,  especially  noticeable  in  tropical 
plants,  and  fruits  and  other  organs,  as  the  radish,  and  beet. 
The  copper  beech  grows  slower  than  our  green-leaved  beech 
and  so  doubtless  in  the  case  of  many  plants  that  have  survived 
not  because  of  their  coloration  but  in  spite  of  it.  In  many 
instances  the  colors  are  due  to  the  acidity  or  alkalinity  of  the 


NATURE   OF   PLANTS  33 

cell  sap.  Red  cells  often  indicate  acidity  and  they  become  blue 
in  weak  potash  solutions.  So  violet  and  blue  indicate  an  al- 
kaline condition  and  such  cells  change  to  red  in  weak  acid  solu- 
tions. This  is  essentially  the  explanation  of  the  changing  tints 
in  the  autumn.  The  changing  character  of  the  cell  sap  is  at- 
tended with  a  gradual  decomposition  of  the  complex  chlorophyll 
and  other  materials  that  results  in  the  formation  of  a  series  of 
substances  that  are  characterized  by  a  variety  of  colors.  Here 
again  the  matter  of  coloration  has  been  over  emphasized.  While 
nitrogen,  phosphorus  and  potassium  are  transported  from  the 
leaves  before  they  fall,  the  coloration  does  not  effect  so  complete 
an  emptying  of  the  leaves  as  was  previously  believed,  if  indeed  it 
is  of  any  service. 

15.  The  Cause  of  Leaf  Arrangement. — How  is  the  perfection 
in  the  arrangement  of  the  leaves  accomplished?  The  leaf  or 
any  other  organ  does  not  remain  in  a  fixed  position  during  its 
growth  and  development  but  owing  to  the  fact  that  first  one  side 
and  then  an  adjoining  side  of  the  organ  is  growing  faster  than 
any  other  part  it  comes  about  that  the  organ  is  bent  from  side  to 
side  by  the  more  rapidly  growing  cells  and  the  apex  of  the  organ 
is  often  caused  to  travel  through  a  rather  irregular  circle.  These 
growing  organs  are  sensitive  to  light,  gravity,  moisture  and  other 
stimuli.  As  a  result  of  these  movements  the  organ  is  brought 
into  relations  with  various  intensities  of  light,  heat,  etc.  In  cer- 
tain positions  the  stimuli  are  not  favorable  and  they  cause  it  to 
grow  away  from  this  position  while  in  other  positions  the  stimuli 
act  in  the  most  favorable  way  and  the  organ  is  stimulated  to  its 
best  growth.  So  during  the  development  of  the  organ  it  is 
brought  into  varied  relations  with  the  forces  which  affect  it  and 
as  a  result  of  this  experience  the  leaf  or  other  organ  is  directed 
by  the  stimuli  and  at  maturity  finally  comes  to  rest  in  a  fixed 
position  that  is  the  most  advantageous.  The  leaf  ordinarily  so 
reacts  to  these  stimuli  that  its  blade  is  exposed  at  right  angles 
to  the  rays  of  light,  but  in  the  iris,  many  grasses  and  rushes 
the  blades  are  nearly  erect.  Quite  a  large  number  of  plants  bear 
their  leaves  edgewise.  The  giant  trees  of  Australia  (Eucalyptus) 
and  the  so-called  compass  plants  are  familiar  examples  (Fig. 


34 


LEAF  ADJUSTMENT 


20).  Possibly  these  blades  are  driven  into  this  position  because 
they  are  more  sensitive  and  the  direct  sunlight  upon  the  broad 
surface  of  the  blade  would  be  injurious.  This  certainly  appears 
to  be  the  case  in  young  leaves  when  emerging  from  the  bud. 
Note  the  character  of  the  foldings  and  the  erect  positions  of  such 
leaves  (Fig.  21).  The  leaves  of  the  horse  chestnut  assume  at 


B 


FIG.  20.  FIG.  21. 

FIG.  20.  Shoot  of  wild  lettuce,  Lactuca,  with  leaves  turned  edgewise  to 
the  light. 

FIG.  21.  Position  of  young  leaves  of  hickory:  A,  scales  of  bud  curving 
back  showing  the  tightly  folded  leaves.  B,  later  stage,  leaves  unfolding  but 
still  erect. 

least  three  different  positions  during  their  growth.  The  folding 
of  such  leaves  and  their  erect  positions  expose  less  surface.  They 
consequently  lose  less  heat  and  moisture,  and  the  delicate  grow- 
ing cells  are  better  protected  against  the  intense  sunlight.  You 
must  notice  as  the  cells  mature  and  become  better  protected  that 
their  irritability  changes  and  they  respond  in  a  different  way 
to  light  and  gravity,  etc.  They  begin  to  unfold  and  turn  away 
from  their  erect  positions  and  at  maturity  assume  a  fixed  posi- 
tion to  light,  etc.,  that  is  quite  different  from  the  original  posi- 


NATURE   OF   PLANTS  35 

tion.  The  position  of  the  leaves  of  our  pines  and  several  other 
evergreens  is  not  controlled  by  light  and  as  a  consequence  they 
assume  a  variety  of  angles  to  the  incident  rays. 

The  leaves  of  many  plants  do  not  have  a  fixed  position  but 
show  more  or  less  motility  during  their  entire  life.  Interesting 
examples  are  seen  in  the  sensitive  plants  and  in  members  of 
the  bean  and  oxalis  families  where  marked  changes  result  in 
the  position  of  the  leaves  from  alterations  in  the  intensity  of 
light,  temperature,  or  moisture.  Many  of  these  plants  fold  their 
leaves  when  the  temperature  falls  at  sundown.  Many  flowers 
close  in  the  same  way.  These  changes  are  popularly  termed  sleep 
movements.  They  are  caused  not  only  by  the  changes  in  the 
intensity  of  the  light  but  also  by  changes  in  temperature  and 
moisture.  In  the  case  of  these  leaves  the  cells  in  the  swollen 
organ  (the  pulvinus)  at  the  base  of  each  leaf  are  kept  full  of 
water  so  long  as  the  temperature  and  external  conditions  exert  a 
suitable  stimulus  upon  the  plant,  but  when  the  conditions  are 
unfavorable  certain  of  the  cells  on  one  side  of  the  pulvinus  lose 
water  and  contract  while  the  cells  on  the  opposite  side  of  the 
pulvinus  remain  rigid;  consequently,  the  leaves  droop  and  fold 
in  various  ways.  This  is  due  in  part  to  the  fact  that  the  con- 
tracted cells  of  the  pulvinus  are  no  longer  capable  of  assisting  in 
the  support  of  the  leaf  and  also  to  the  fact  that  the  rigid  cells  con- 
tinue to  exert  a  pressure  which  bends  the  pulvinus  towards  the 
side  where  the  contracted  cells  are  situated  (Fig.  22).  This  fold- 
ing of  the  leaves  materially  reduces  the  area  exposed  to  the 
atmosphere  and  consequently  will  lessen  transpiration.  Many  of 
our  sensitive  clovers  (Meibomia,  Lespedeza),  sensitive  peas  (Cas- 
sia), sensitive  plant  (Mimosa),  are  able  to  check  the  excessive 
loss  of  water  on  hot  dry  days  by  the  folding  of  their  leaves.  As 
soon  as  the  loss  of  water  becomes  detrimental  some  of  the  leaves 
begin  to  fold  and  the  reduction  of  leaf  surface  continues  until  the 
loss  of  water  by  transpiration  is  met  by  root  absorption.  Very 
commonly  leaves  that  do  not  have  this  power  of  adjustment  are 
able  to  accomplish  the  same  results  by  a  rolling  of  the  leaf.  This 
is  particularly  noticeable  among  the  grasses,  which,  on  hot,  dry 
days,  roll  up  their  leaves  so  tightly  as  to  change  the  appear- 


36  NATURE   OF  THE   EPIDERMIS 

ance  of  the  plants.  Observe  a  corn  field  during  a  dry  period. 
The  interesting  feature  of  all  this  is  that  these  reactions  and 
adjustments  are  purposive  but  not  intelligent.  The  movements 
are  not  called  forth  by  consciousness  but  by  stimuli  to  which  the 
irritable  living  substance  is  attuned  or  sensitive. 

16.  The  "Significance  of  Certain  External  Leaf  Structures. — 
Many  features  connected  with  the  structure  of  the  leaf  furnish 
admirable  illustrations  of  the  fitness  of  the  leaf  for  the  perform- 


FIG.  22.  Leaf  position  of  the  sensitive  plant,  Mimosa:  A,  in  light.  B,  in 
darkness.  The  same  movements  occur  as  a  result  of  unfavorable  temperatures 
and  humidity. 

ance  of  its  work.  The  stimulus  of  light  and  moisture  have  a 
marked  influence  upon  the  external  structure  and  form  of  leaves. 
The  epidermis  is  strikingly  modified  by  such  forces.  In  shade 
plants  the  epidermis  consists  of  a  rather  delicate  layer  of  cells 
with  very  thin  cuticle.  This  gives  sufficient  protection  to  such 
plants,  but  in  leaves  exposed  to  intense  sunlight  and  a  hot  and 
dry  air,  the  epidermal  cells  become  greatly  thickened  and  often 
of  two  or  more  rows,  while  the  cuticle  may  often  form  the  larger 
part  of  the  outer  cell  wall  or  even  extend  in  between  the  cells 
(Fig.  23,  c).  Cell  walls  that  are  filled  with  cutin  in  this  manner 
are  said  to  be  cutinized.  This  development  of  cutin  gives  the 
tough  leathery  aspect  to  many  leaves.  Such  features  are  partic- 
ularly noticeable  in  desert  plants,  in  long-lived  leaves  of  many 


NATURE   OF   PLANTS  37 

evergreens,  as  laurels  and  rhododendrons,  in  the  needles  of  cone- 
bearing  trees,  and  in  all  plants  that  are  exposed  to  the  hot,  dry  air 
of  summer  or  the  drying  winds  of  winter.  Such  leaves  may  be 
further  protected  and  strengthened  by  thick-walled  elongated 
cells,  stereome  (Fig.  23,  sf).  Coatings  of  wax,  mucilage  and 
lime  are  also  frequently  developed  upon  the  cuticle  to  further 
reinforce  the  impermeability  of  the  epidermis. 

The  stomata  are  also  so  developed  as  to  meet  the  conditions 
under  which  the  plant  grows.  In  dorsiventral  leaves  they  are 
more  numerous  on  the  under  side  of  the  leaves  because  they  are 
less  liable  to  be  filled  with  water  by  rains  and  with  dust  which 
would  prevent  the  interchange  of  gases.  The  plugging  of  the 


FIG.  23.  Cross-section  of  the  outer  cells  of  a  leaf  of  pine,  showing  the  firm 
character  of  the  outer  cells  of  the  tough  leaf:  s,  stoma;  e,  epidermis;  c,  cuticle; 
st,  stereome;  m,  mesophyll  cells. 

stomata  by  dust  is  one  of  the  causes  of  the  sickliness  of  plants  in 
homes.  The  arrangement  of  the  stomata  on  the  under  side  of  the 
leaf  is  also  of  especial  advantage  because  the  direct  light  does  not 
fall  upon  them  and  cause  an  excessive  loss  of  water.  The  stomata 
of  floating  leaves,  however,  are  upon  the  upper  surface  and  their 
stoppage  with  water  is  prevented  by  waxy  coatings,  as  can  be 
easily  demonstrated  by  dipping  a  leaf  of  a  water  lily  or  spatter 
dock  in  the  water.  When  the  leaf  is  removed  the  water  runs 
off  of  the  waxed  surface  without  wetting  it.  Some  leaves  are 
more  or  less  erect,  as  the  cattails,  rushes  and  grasses,  and  these 
have  the  stomata  developed  more  or  less  evenly  on  both  surfaces. 
Doubtless  the  intense  light  of  midday  is  not  beneficial  to  these 
leaves  and  the  blade  of  the  leaf  is  consequently  placed  parallel 


38  LOCATION   OF    STOMATA 

with  the  sun's  rays.  This  arrangement  of  the  leaf  permits  a 
direct  illumination  of  the  leaf  only  in  the  morning  and  afternoon 
when  the  intensity  of  the  light  is  feebler. 

The  position  and  character  of  the  stomata  in  relation  to  exter- 
nal conditions  show  many  interesting  relations.  Plants  living  in 
the  shade  or  in  the  presence  of  an  abundant  soil-moisture  develop 


FIG.  24.     Cross-section  of  a  leaf  of  the  inch  plant,  Tradescantia,  showing 
the  delicate  character  of  the  cells  and  the  raised  stoma  of  this  shade  plant. 

the  stomata  on  a  level  with  the  leaf  surface  (Fig.  24),  because 
there  is  no  necessity  of  conserving  the  water  supply.  For  the 
same  reason  some  aquatics  have  lost  altogether  the  power  of 
closing  their  stomata.  On  the  other  hand  plants  that  are  exposed 
to  arid  conditions  or  drying  winds  develop  the  stomata  well 
below  the  surface  of  the  leaf,  as  in  the  cactus  and  in  the  needles 
of  conifers  (Fig.  23,  s),  or  in  furrows,  as  in  certain  grasses,  or 
at  the  bottom  of  minute  pores,  as  in  the  oleander.  These  de- 
pressions remove  the  stomata  from  the  dry  winds  and  prevent 
the  direct  contact  of  the  moist  air  in  the  leaf  with  the  dry  atmos- 
phere. The  pores  in  the  oleander  contain  hairs  which  would 
check  transpiration  just  as  a  plug  of  cotton  in  the  neck  of  a 
flask  would  lessen  the  evaporation  of  the  fluid  in  the  flask.  The 
chief  purpose  of  this  arrangement,  however,  is  to  prevent  the 
stopping  of  the  stomata  with  water.  This  plant  grows  naturally 
along  the  banks  of  streams  where  it  is  subject  almost  nightly  to 
heavy  dews.  When  the  stomata  become  filled  with  dew  it  re- 


NATURE  OF   PLANTS  39 

quires  several  hours  of  sunshine  to  drive  the  moisture  from  these 
capillary  openings.  Consequently  during  this  time  there  could 
be  no  interchange  of  gases  and  the  vital  functions  of  the  leaves 
would  be  largely  stopped.  Many  devices  appear  that  prevent 
water  entering  the  stomata.  Attention  has  been  called  to  the 
development  of  stomata  on  the  under  surface  of  leaves  and  to 
waxy  coatings  as  protective  features  of  this  nature.  Every  one 
is  familiar  with  the  waxy  appearance  or  bloom  of  many  fruits, 
as  the  plum;  or  of  leaves,  as  the  cabbage;  and  of  stems,  as  the 
raspberry.  Water  is  unable  to  spread  over  the  wax  and  enter 
the  stomata  because  a  thin  layer  of  air  clings  to  it.  The  silvery 
appearance  of  many  grass  leaves  and  of  the  jewel- weed  when 
immersed  in  water  is  due  to  this  thin  film  of  air  which  reflects 
the  light.  Accordingly  the  water  does  not  really  touch  the  leaf 
and  you  notice  that  it  is  quite  dry  when  it  is  removed  from  the 
water  save  for  a  few  drops  which  readily  run  off.  Protection 
against  wetting  is  also  obtained  by  the  development  of  coatings 
of  hairs.  This  is  noticeable  in  many  Alpine  and  polar  plants 
where  they  are  constantly  exposed  to  heavy  fogs,  dews,  and  rains. 
Plant  hairs  do  not  always  serve  this  purpose.  Desert  plants 
and  those  living  in  dry  localities  are  frequently  characterized  by 
hairy  coatings.  Such  plants  are  frequently  termed  xerophytes 
in  contradistinction  to  aquatics  or  hydrophytes  and  to  plants 
living  in  moderately  moist  soils,  which  are  called  mesophytes. 
The  hairy  coatings  of  xerophytes  give  them  their  familiar  silvery 
or  hoary  appearance  as  seen  in  the  sage-brush,  dusty  miller,  and 
mullein.  The  hairs  are  usually  empty  tubes  or  cells  which  re- 
flect a  large  portion  of  the  intense  light  and  heat  and  consequently 
materially  lessen  the  loss  of  water,  just  as  straw  sprinkled  upon 
the  ground  keeps  the  earth  below  moist  and  cool.  In  many  in- 
stances plant  hairs  develop  mucilage  or  resinous  secretions  which 
protect  as  with  a  varnish  growing  regions  as  buds  or  young  shoots 
and  leaves  of  birches,  alders,  poplars,  and  peach.  In  this  con- 
nection it  should  be  noted  that  dense  hairy  coatings  reduce  the 
loss  of  water  only  when  the  plant  is  exposed  to  air  currents — 
not  in  still  air.  Cutin,  wax  and  resinous  deposits  retard  trans- 
piration under  all  conditions.  Oil  and  poisonous  substances  are 


40  ARRANGEMENT   OF  THE   MESOPHYLL 

also  formed  which  repel  by  disagreeable  odors  or  tastes  the 
attacks  of  animals.  The  nettles  are  shunned  because  of  the 
burning  sting  produced  by  the  plant  hairs.  The  upper  portion 
of  these  hairs  (Fig.  25)  is  practically  a  delicate  glass  tube  since 
the  walls  are  filled  with  silica.  The  basal  portion  of  the  hair  is 
composed  of  soft,  yielding  cellulose  so  that  the  fluid  collecting 
in  the  hair  distends  this  portion  of  it.  The  knob  at  the  end  is 
fastened  to  the  hair  by  so  thin  a  ring  of  wall,  as  seen  in  Fig.  25, 
B,  that  the  least  touch  breaks  it  off.  In  this  way  the  hair  is 


FIG.  25.     Stinging   hair   of   nettle:   A,    portion   of   epidermis   bearing   hair. 
B,  tip  of  hair  enlarged,  showing  easily  detachable  knob. — I.  D.  Cardiff. 

transformed  into  a  veritable  hypodermic  needle  which  easily 
punctures  the  skin  while  the  distended  basal  portion  contracts  and 
forces  out  some  of  the  fluid.  The  burning  sensation  or  sting 
that  immediately  follows  the  puncture  of  the  skin  is  caused  by 
the  injection  of  formic  acid  while  a  variety  of  other  poisons  pro- 
duce the  subsequent  irritations.  These  poisons  are  so  powerful 
in  some  of  the  East  Indian  nettles  as  to  produce  serious  results, 
even  a  tetanus. 

17.  Significance  of  Certain  Internal  Leaf  Structures. — The 
changes  or  modifications  produced  in  the  chlorenchyma  by  vari- 
ous stimuli  are  quite  as  striking  as  in  the  case  of  the  epidermis. 
The  palisade  cells  are  an  excellent  illustration  of  this  point. 
These  cells  are  developed  as  a  result  of  the  stimulating  influence 
of  light  (Fig.  26).  The  photosynthetic  activity  caused  by  the 
light  increases  the  pressure  of  the  fluids  in  the  cells.  This 


NATURE  OF   PLANTS 


tends  to  round  them  out.  They  are  unable  to  expand  laterally 
owing  to  the  adjacent  cells  but  are  free  to  extend  towards  the 
looser  cells  below,  and  so  they  become  longer  and  palisaded. 
Leaves  growing  in  the  deep  shade  show  little  indication  of  palisade 


FIG.  26.  FIG.  27. 

FIG.  26.  Section  of  a  leaf  of  Rhododendron.  Note  the  compact  palisade 
tissue  which  results  from  intense  light. 

FIG.  27.  Section  of  a  leaf  of  skunk  cabbage,  Spathyema.  Note  the  poorly 
developed  palisade  tissue  and  the  loose  arrangement  of  the  cells  of  this  plant 
which  lives  in  moist,  shaded  places. 

structure.  Shade  and  moisture  plants  are  not  obliged  to  conserve 
the  amount  of  water  received  and  these  two  forces,  feeble  light 
and  moisture,  produce  a  larger  and  looser  arrangement  of  tissues 
which  is  favorable  to  an  interchange  of  gases  (Fig.  27).  Par- 
ticularly is  this  noticeable  in  aquatics  as  in  the  water  lilies  and 
many  rushes,  etc.,  where  the  tissue  is  loose  and  spongy  and 
permits  a  ready  circulation  of  gases  from  the  leaves  to  the  roots 
and  to  all  parts  of  the  plant  body  even  when  submerged.  This 
loose  arrangement  of  tissues  also  renders  water  plants  very 
buoyant  and  consequently  they  are  less  liable  to  injury  from 
the  currents  of  the  water.  Aquatic  plants  have  little  need  of 
strengthening  and  conducting  tissues  because  they  are  supported 
4 


42  LEAF   FALL 

by  the  water.  Neither  do  they  require  elaborate  bundles  for  the 
conduction  of  the  crude  foods  which  may  be  absorbed  by  nearly 
all  parts  of  the  plant  body.  We  now  understand  why  shade  leaves 
are  thin  and  broad  and  soft  whereas  leaves  of  desert  plants  and 
those  exposed  to  severe  drying  winds  of  summer  or  winter  are 
thick,  compact,  firmer  and  often  leathery  and  hairy. 

One  of  the  most  interesting  adaptive  features  of  leaves  is  seen 
in  the  leaf  fall  of  our  deciduous  trees  and  shrubs.  In  the  tropics 
the  leaves  remain  on  the  perennial  plants  often  for  long  periods, 
but  in  temperate  climates  the  severe  winters  necessitate  the 
annual  dropping  of  the  leaves  except  in  a  comparatively  few 
evergreens  where  the  thick  leathery  leaves  are  able  to  endure 
such  conditions.  In  our  deciduous  plants,  when  the  conditions 
are  no  longer  favorable  for  the  performance  of  leaf  work,  the 
cells  at  the  base  of  the  petiole  begin  to  divide  and  a  delicate 
layer  of  cells,  the  separating  layer,  is  formed  across  the  petiole 
(Fig.  28).  Various  conditions  induce  this  growth  for  there  is 
more  or  less  leaf  fall  at  all  seasons  of  the  year.  Doubtless  lack 
of  nutriment  has  much  to  do  with  it.  If  a  leafy  branch  of  horse 
chestnut  is  placed  between  moist  paper  in  a  few  days  the  separa- 
ting layer  will  have  formed  and  the  leaf  will  have  dropped  from 
the  branch.  The  innermost  delicate  cells  of  the  separating  layer 
may  become  changed  into  cork  cells  just  before  or  directly  after 
leaf  fall  while  the  outer  cells  of  this  layer  may  break  down 
through  mucilaginous  modification  or  they  may  become  rounded 
off  so  that  water  collects  in  the  intercellular  spaces.  Thus  the 
leaf  is  attached  to  the  stem  chiefly  by  the  veins  since  the  delicate 
separating  layer  offers  little  support.  If  now  the  water  in  the 
intercellular  spaces  of  the  separating  layer  should  freeze,  the 
expansion  of  the  water  as  it  freezes  would  so  rupture  the  remain- 
ing tissue  that  we  would  have  the  familiar  sight  of  the  leaves 
falling  in  a  shower  in  the  morning  after  a  frost — either  of  their 
own  weight  or  with  the  slightest  breeze.  It  will  also  be  noticed 
that  owing  to  the  formation  of  the  cork  layer  or  the  drying  up 
of  the  delicate  cells  of  the  separating  layer,  the  scar  formed  by 
the  fall  of  the  leaf  is  nicely  healed  and  closed  against  any  loss  of 
fluids  or  the  entrance  of  any  organism  (Fig.  28,  C). 


NATURE   OF   PLANTS 


43 


The  casting  off  of  the  leaves  reduces  the  area  of  the  plant 
that  is  exposed  to  the  unfavorable  conditions  to  a  minimum. 
This  reduction  of  the  surface  is  effected  in  one  way  or  another 
by  a  great  variety  of  plants.  Much  of  our  spring  vegetation 
is  possible  by  reason  of  it.  The  spring  beauties,  anemones,  fawn 
lilies,  iack-in-the-pulpits,  etc.,  practically  complete  their  growth 


FIG.  28.  Leaf  fall:  A,  branch  of  horse-chestnut  showing  scar  formed  by 
the  fall  of  the  leaf.  The  dots  on  the  scar  show  the  position  of  the  vascular 
bundles  that  are  finally  broken  by  the  weight  of  the  leaf.  At  the  left  the 
base  of  the  petiole  is  shown.  B,  diagram  of  a  section  through  a  twig  of  hickory 
— s,  separating  layer  at  base  of  petiole;  v,  vascular  bundles.  C,  enlarged 
view  of  the  separating  layer — c,  cork  cells  that  heal  the  wound  caused  by  the 
fall  of  the  leaf.  The  granular  cells  are  the  outer  region  of  the  separating 
layer,  and  they  are  beginning  to  break  down  as  seen  in  the  upper  part  of  the 
figure,  at  x,  thus  causing  the  fall  of  the  leaf;  st,  cells  of  the  stem  containing 
starch. 

before  the  larger  summer  forms  appear.  During  this  short  period 
sufficient  food  is  manufactured  by  the  leaves  to  mature  the  seeds 
and  fill  the  storage  organs  in  the  underground  bulbs  and  stems. 


44  NATURE   OF   DESERT   PLANTS 

When  the  larger  forms  appear  later  in  the  season  these  plants 
are  overshadowed,  their  leaves  wither,  and  they  are  reduced  to 
the  small  bulbs  and  stems  hidden  in  the  earth.  In  this  condition 
they  remain  until  the  next  spring  when  the  abundance  of  stored 
food  enables  them  to  complete  their  growth  before  other  com- 
peting forms  appear. 

This  ability  to  reduce  the  surface  of  the  plant  body  makes 
possible  the  existence  of  much  of  the  plant  life  of  arid  and  desert 
regions.  During  the  short  rainy  seasons  the  stored  foods  enable 
the  plant  to  quickly  put  forth  the  leaves  which  in  turn  manu- 
facture the  foods  which  are  stored  in  seeds,  stem  and  roots- 
Then  practically  all  trace  of  these  plants  is  destroyed  by  the 
drought.  Only  the  seeds  or  a  greatly  reduced  portion  of  the 
plant  remain  alive  and  this  is  often  further  protected  by  being 
hidden  in  the  ground.  A  return  of  favorable  conditions  quickly 
awakens  these  plants  to  growth.  Mr.  Dan  Beard  relates  an 
interesting  experience  in  the  desert  regions  of  Texas  that  illus- 
trates this  feature  of  plant  life.  At  the  close  of  the  dry  season 
this  territory  was  a  bare,  sun-cracked  plain  swept  by  hot,  dry 
winds.  Not  a  green  leaf  was  to  be  seen.  Heavy  rains  trans- 
formed this  level  tract  of  land  into  an  inland  sea.  In  a  few  days 
the  water  disappeared  and  one  morning  he  was  surprised  to  see 
green  blades  appearing  everywhere  and  in  a  few  days  the  barren 
earth  was  covered  with  an  almost  tropical  profusion  of  vegeta- 
tion ,of  bright  flowers.  Many  desert  plants  to  be  sure  develop 
permanent  aerial  parts  but  here,  too,  the  most  striking  feature 
is  the  extreme  reduction  of  the  organs.  Compare  the  areas  of 
the  Spanish  bayonet,  aloes,  grease  wood  and  sage  with  that  of 
our  leafy  plants.  The  cactus  represents  one  of  the  extreme  forms 
of  reduction.  The  leaves  have  been  dispensed  with  entirely  and 
are  not  represented  save  possibly  by  the  spines.  Consequently 
the  work  of  the  leaves  devolves  upon  the  stems.  While  desert 
plants  receive  very  meager  amounts  of  water,  this  is  so  effectually 
conserved  that  the  plants  are  usually  rich  in  water.  Drinking 
water  is  frequently  obtained  in  desert  regions  from  cacti  by 
pounding  up  the  pulpy  interior  and  squeezing  out  the  water. 
Animals  are  well  aware  of  this  rich  storehouse  and  will  eagerly 


NATURE  OF   PLANTS  45 

devour  the  cacti  after  the  spines  are  burned  off.  The  develop- 
ment of  these  storage  cells  accounts  for  the  fleshy  character  of 
many  alkaline,  saline,  and  desert  plants,  many  of  which  depend 
upon  organic  compounds  lodged  in  their  cells  for  the  retention  of 
water  rather  than  upon  the  development  of  cutin.  Curiously 
enough  there  are  many  examples  of  xerophytic  plants  living  in 
bogs  and  marshes,  as,  for  example,  the  rushes  and  sedges,  the 
horse  tail  ferns  (Equisetum),  the  lamb  kill  (Kalmia)  and  leather 
leaf  (Chamaedaphne) ,  etc.  The  cause  of  the  association  of  these 
plants  with  aquatic  forms  is  not  known.  In  exposed  moors  and 
heaths  these  reduced  forms  would  have  decided  advantage 
because  of  their  protection  against  drying  winds.  Our  sedges  and 
rushes  are  exposed  to  very  intense  heat  and  light  which  may 
possibly  cause  so  heavy  a  transpiration  that  these  plants  are  not 
able  to  meet  the  loss  by  root  absorption.  This  is  the  more 
probable  because  in  many  instances  the  absorption  of  water  by 
plants  living  under  such  conditions  is  slow  owing  to  lack  of 
oxygen  and  to  the  concentration  of  the  water  in  which  the  roots 
grow. 


CHAPTER   II 

THE   ROOT 

18.  Character  of  Primitive  Plants. — The  first  forms  of  life 
upon  the  earth  were  doubtless  unattached  unicellular  plants  that 
lived  in  the  water  or  moist  places.     All  the  varied  functions  of 
the  plant  body,  as  the  absorption  of  gases  and  material  from 
the  soil  and  the  manufacture  of  organic  compounds,  etc.,  were 
performed  alike  by  each  cell.     This  was  easily  accomplished 
owing  to  the  simplicity  of  the  organism  and  also  because  the  plant 
was  surrounded  by  water  which  contained  all  the  substances  re- 
quired in  the  construction  of  foods.     Perhaps  the  first  change 
produced  in  these  plants  by  their  surroundings  was  a  modification 
of  a  part  of  a  cell  so  that  it  served  as  an  organ  of  attachment,  or 
root,  anchoring  the  plant  to  the  substratum.     As  the  plant  con- 
tinued to  change  and  become  more  complex,  and  especially  as  it 
became  more  and  more  subject  to  drier  conditions,  special  organs 
for  the  absorption  of  materials  became  a  necessity.     This  change 
from  an  aquatic  to  a  terrestrial  life  finally  left  the  root  alone  in 
touch  with  the  crude  materials.     Thus  the  root  which  at  first  was 
only  an  anchoring  organ,  came  later  to  function  also  as  the  prin- 
cipal absorbing  organ  of  the  plant.     Next  to  the  leaf  the  root 
commands  our  attention  because  of  its  fitness  for  the  accomplish- 
ment of  this  work.     Practically  all  the  water  required  by  land 
plants,  and  all  the  elements,  save  carbon  and  oxygen,  utilized  in 
the  construction  of  foods  are  absorbed  by  the  roots. 

19.  Root  Hairs. — We  will  first  be  interested  to  examine  the 
structure  and  nature  of  the  absorbing  apparatus  of  the  root.     If 
seeds  of  radish,  mustard,  or  other  plants  are  germinated  upon 
moist  sand  or  moist  blotting  paper  it  will  be  seen  when  the  roots 
have  attained  a  length  of  several  cm.  that  a  portion  of  each  root 
is  covered  with  delicate  hairs  (Fig.  29,  A).     There  are  several 
interesting  features  about  these  root  hairs.     In  the  first  place  they 
are  extremely  delicate  tubular  outgrowths  of  the  epidermal  cells, 

46 


NATURE  OF  PLANTS 


47 


scarcely  more  than  one  millimeter  in  length  (Fig.  30).  Note 
that  they  always  begin  to  grow  a  few  mm.  back  from  the  root 
tip  and  at  the  opposite  end  of  the  zone  of  root  hairs  the  tubes  are 
withering  or  dying  off,  leaving  the  older  portion  of  the  root 
covered  only  with  the  epidermis.  If  a  root  is  marked  into  2  mm. 
spaces  with  India  ink  it  will  be  found  after  12  to  15  hours  that 
only  the  portion  below  the  root  hairs  is  elongating.  The  first 
space  from  the  tip  has  grown,  very  little,  the  second  and  third 


FIG.  29.  FIG.  30. 

FIG.  29.  The  absorbing  surface  of  roots:  A,  seedling  of  mustard  showing 
extent  of  surface  covered  by  root  hairs.  B,  an  older  branching  root,  the 
shaded  areas  near  the  tip  are  due  to  particles  of  earth  clinging  to  the  root  hairs. 
The  remaining  portions  of  the  root  are  free  hairs  and  take  little  or  no  part  in 
absorption. 

FIG.  30.  Origin  and  character  of  root  hairs:  A,  first  appearance  of  a  root 
hair  due  to  the  pushing  out  or  growth  of  a  definite  portion  of  the  surface  of  an 
epidermal  cell.  B,  older  hairs  that  have  become  irregular  through  coming 
in  contact  with  soil  particles. 

much  more,  and  in  the  other  spaces  there  is  a  gradual  retarding 
of  the  growth  until  8  to  12  mm.  from  the  tip  elongation  has 
ceased.  It  is  at  the  point  where  elongation  has  ceased  that  the 
root  hairs  begin  to  appear,  but  the  total  number  existing  on  the 
root  at  any  one  time  does  not  materially  vary  owing  to  the  fact 
that  they  are  dying  off  farther  back  on  the  root  with  the  same 
rapidity  that  characterizes  their  formation  near  the  tip  (Fig. 
29,  B).  Before  the  significance  of  these  peculiar  features  can  be 


48  NATURE   OF  SOILS 

understood  it  will  be  necessary  to  consider  the  nature  of  soils 
and  the  relation  of  the  substances  absorbed  by  the  plant  to  the 
soils. 

20.  The  Nature  of  Soils. — The  soil  is  composed  of  minute 
particles  derived  from  decaying  rocks  and  of  organic  particles 
or  humus  derived  from  decaying  plants  and  animals.  The 
mineral  particles  furnish  several  of  the  important  crude  plant 
foods,  as  calcium,  magnesium,  potassium,  phosphorus,  sulphur, 
iron  and  small  amounts  of  other  elements.  These  substances 
occur  in  great  abundance,  comprising  over  thirteen  per  cent,  of 
the  earth's  crust.  It  is  a  singular  fact  that  two  of  the  most 
abundant  substances,  silicon  and  aluminium,  are  utilized  by 
only  comparatively  few  plants,  although  it  will  be  seen  later  on 
that  the  latter  element  is  indirectly  of  great  service  to  plant  life. 
The  remaining  elements  used  by  the  plant,  oxygen,  nitrogen, 
carbon,  hydrogen,  are  equally  abundant.  The  first  three  may  be 
obtained  from  the  air  while  the  hydrogen  is  a  constituent  of  water 
and  other  compounds.  It  is  an  interesting  fact,  however,  if 
rocks  containing  all  these  mineral  elements  should  be  ground  up 
into  the  form  of  a  good  soil  and  the  other  necessary  substances  be 
added  thereto  that  it  would  not  support  plant  life  at  all.  The 
reason  is  that  these  elements  exist  in  the  rocks  in  a  form  that  the 
plants  can  not  utilize.  They  are  largely  combined  with  an  ele- 
ment silicon,  of  which  substance  pure  sand  or  quartz  is  largely 
composed.  These  compounds  are  termed  silicates.  The  plant 
can  only  utilize  these  elements  when  they  are  combined  with  some 
element,  other  than  silicon,  and  to  which  oxygen  has  been  added. 
The  potassium,  magnesium,  etc.,  must  be  combined  with  nitro- 
gen, phosphorus,  sulphur  or  carbon  to  which  oxygen  is  also  added 
in  order  to  make  a  substance  that  the  plant  can  utilize.  These 
crude  foods  of  the  plant  are  termed  nitrates,  phosphates, 
phates  and  carbonates.  It  is  only  in  this  form  that  the  necessary 
elements  can  be  absorbed,  silicates  are  rarely  of  service.  The 
question  naturally  arises:  how  do  the  silicates  become  trans- 
formed into  suitable  compounds  and  how  do  the  rocks  become 
changed  into  the  form  of  a  soil.  Several  factors  are  concerned 
in  the  transformation  and  the  reactions  are  complicated  and 
very  slowly  effected. 


NATURE  OF   PLANTS  49 

Water  is  the  most  important  force  in  starting  these  changes. 
It  dissolves,  that  is  removes,  several  of  the  elements  that  appear 
in  the  silicates.  Note  also  when  water  has  taken  up  certain  of 
these  elements  and  compounds  that  its  dissolving  power  is  greatly 
increased.  For  example  when  water  has  absorbed  carbon  dioxide 
its  power  of  dissolving  calcium  is  increased  25  times.  Further- 
more the  removal  of  an  element  by  the  water  means  that  new 
chemical  compounds  are  formed  and  these  are  frequently  of  such 
a  nature  as  to  lead  to  further  chemical  reactions.  We  say  that 
the  dissolving  action  of  water  upsets  the  chemical  stability  of 
the  substances  composing  the  rocks.  Certain  gases  of  the  air> 
particularly  oxygen,  also  enter  into  union  with  some  of  the  rock 
elements,  making  them  more  soluble.  So  you  can  think  of  a 
series  of  chemical  struggles  going  on  between  the  silicon  and 
other  elements  and  compounds  for  the  possession  of  the  potas- 
sium, calcium,  etc.,  and  that  in  this  way  these  latter  elements 
are  slowly  removed  from  the  silicon  and  united  into  new  com- 
pounds suitable  for  plants. 

The  removal  of  these  elements  from  the  rock  minerals  forms 
cavities  and  as  a  consequence  the  rock  tends  to  become  honey- 
combed and  finally  begins  to  crumble  and  so  approaches  a  soil 
condition.  Two  forces  are  therefore  involved  in  soil  making: 
the  one  chemical  which  is  termed  decomposition  and  the  other 
mechanical,  termed  disintegration.  Disintegration  or  the  crumb- 
ling of  the  rocks  is  hastened  by  a  number  of  factors,  especially 
the  force  of  running  water.  The  power  of  water  to  break  up  the 
rock  substance  and  distribute  the  particles  thus  formed  is  one 
of  the  most  important  factors  in  soil  formation.  Changes  in 
temperature  also  constantly  cause  the  contraction  and  expansion 
of  the  rock  material  and  of  the  water  occupying  any  of  the  spaces 
in  the  rock  and  so  assist  in  the  disintegration.  Finally  the  force 
of  the  wind  and  the  grinding  action  of  moving  ice  are  important 
factors  in  wearing  away  the  rock  surface  and  reducing  it  to  a  soil 
condition.  By  these  slow  and  intricate  processes  compounds 
for  plant  uses  are  formed  and  the  rocks  are  reduced  to  the  condi- 
tion of  a  soil.  Such  a  soil,  however,  does  not  contain  sufficient 
amounts  of  the  crude  food  material  to  support  the  ordinary 


50  PROPERTIES   OF  SOILS 

plant.  So  we  find  very  simple  plants,  as  the  lichens  and  certain 
mosses,  occupying  such  primitive  soils.  They  are  able  to  live 
upon  such  soils  because  they  are  small  and  slow  growing  and 
therefore  demand  meager  amounts  of  the  crude  foods.  Note 
the  character  of  the  first  vegetation  that  appears  on  the  slopes 
of  mountains  below  the  solid  rock.  These  simple  plants,  how- 
ever, are  very  important  factors  in  effecting  further  decomposition 
of  the  minerals  in  the  soil.  Through  the  chemical  activity  of 
their  absorbing  organs  and  especially  through  the  decay  of  the 
dead  parts  of  these  plants  new  compounds  are  formed  so  that 
more  elaborate  plants,  as  certain  ferns,  are  now  able  to  live  upon 
the  soil.  These  latter  plants  in  turn  carry  on  the  work  until 
the  rock  material  is  sufficiently  changed  to  make  a  good  soil. 
Do  not  imagine  that  the  work  of  soil  formation  stops  here.  You 
are  to  think  of  every  growing  plant  performing  the  same  work 
as  noted  in  the  lichens  and  mosses. 

The  soil  is  one  of  the  most  remarkable  things  in  this  world  of 
ours  and  we  must  consider  two  of  its  more  important  features. 
The  first  of  its  important  characteristics  is  its  power  to  retain 
moisture.  Water  exists  in  the  soil  in  three  forms, — as  hygro- 
scopic, as  capillary  and  as  gravitational  water.  The  first  form 
is  the  water  that  condenses  from  the  atmosphere  as  extremely 
delicate  films  about  the  air-dry  soil  particles.  This  water  is  not 
available  to  the  plant  because  it  is  held  so  firmly  by  the  soil 
particles.  It  comprises  only  a  small  per  cent,  of  the  total  water 
in  the  soil.  The  capillary  water  also  exists  as  films  about  the 
soil  particles  but  it  is  not  retained  so  tenaciously  and  it  is  this 
water  that  furnishes  the  chief  supply  to  land  plants.  Capillary 
water  may  amount  to  10  per  cent,  of  the  dry  weight  of  coarse 
sand  up  to  25  per  cent,  in  clay  soils.  Gravitational  water  is  not 
retained  by  the  soil  particles  and  it  is  therefore  free  to  run  down 
through  the  pores  and  spaces  in  the  soil  under  the  control  of 
gravity.  This  water  in  the  upper  portions  of  the  soil  is  not  of 
value  to  the  majority  of  plants  because  it  fills  the  pore  spaces  in 
the  soil  and  so  deprives  the  roots  of  the  air  which  is  necessary 
to  their  existence.  When  gravitational  water  exists  a  few  feet 
below  the  surface  it  often  becomes  of  great  value  as  a  source  of 
supply  for  the  capillary  water. 


NATURE   OF   PLANTS  51 

Let  us  now  consider  how  the  root  is  related  to  the  soil  water. 
You  have  seen  that  the  soil  particles  have  an  attraction  for  water. 
This  attraction  is  due  to  a  force  termed  surface  tension.  Each 
soil  particle  has  a  definite  surface  tension  or  capacity  for  retaining 
water  that  is  dependent  upon  the  extent  of  its  surface.  The 
retention  of  the  hygroscopic  water  uses  up  a  small  portion  of  the 
particle's  capacity  and  then  it  adds  capillary  water  until  no  more 
can  be  retained.  We  say  that  the  particle  is  in  a  saturated  state. 
It  is  evident  that  the  last  water  added  to  the  particle  will  not  be 
held  very  firmly  and  it  is  this  water  that  the  root  hair  can  readily 
absorb.  But  now  follows  a  very  remarkable  movement  of 
water.  No  sooner  has  a  particle  lost  some  of  its  water  than  it  is 
able  to  replenish  its  supply  from  adjacent  particles  that  are  in  a 
saturated  state.  This  transfer  of  water  from  particle  to  particle 
goes  on  for  surprising  distances,  depending  upon  the  amount  of 
the  surface  tension.  So  we  find  particles  four  to  six  feet  away 
from  the  root  hair  contributing  a  portion  of  their  capillary  water 
to  replace  the  water  thus  absorbed.  So  you  can  think  of  the 
capillary  water  moving  as  films  from  particle  to  particle  towards 
the  points  of  absorption.  It  is  for  this  reason  that  the  gravita- 
tional water  may  be  of  great  value.  If  it  is  only  a  few  feet  below 
the  absorbing  roots  it  will  furnish  a  constant  supply  of  capillary 
water  to  the  overlying  soil  particles.  So  one  of  the  most  im- 
portant features  about  a  soil  is  the  size  of  its  particles.  The 
term  texture  refers  to  this  condition  of  a  soil  and  we  speak  of  fine 
or  coarse  textured  soils.  The  finer  the  particles  the  greater  the 
surface  exposed  in  a  given  unit  of  space;  and  consequently  the 
greater  the  surface  tension  and  water-holding  capacity.  A 
cubic  foot  of  coarse  sand  can  not  retain  as  much  water  as  a  cubic 
foot  of  clay  because  the  clay  particles  are  200  times  smaller  than 
the  sand  particles.  A  fine  textured  soil  may  lose  some  of  its 
capacity  to  retain  water  owing  to  the  compacting  of  its  particles 
into  larger  ones.  It  is  evident  that  two  separate  particles  possess 
more  surface  area  than  when  combined  into  one  particle.  This 
relation  of  the  particles  to  one  another  is  termed  structure  and 
we  see  that  soil  structure  is  of  equal  importance  with  soil  texture 
Now  we  can  understand  the  chief  advantage  of  tillage.  Plowing 


52  SOIL  ABSORPTION 

and  harrowing  break,  up  the  compact  structure  and  tend  to  keep 
the  soil  in  a  fine  textural  condition.  It  is  surprising  to  note  the 
amount  of  surface  exposed  by  the  particles  of  various  types  of 
soils.  A  coarse  sand  where  the  particles  range  from  .1  to  .5  mm. 
in  diameter  has  no  less  than  40,000  sq.  ft.  of  surface  exposed  in 
each  cu.  ft.  of  soil  and  a  clay  soil  where  the  particles  are  .005  mm. 
in  diameter  or  smaller  has  over  140  thousand  sq.  ft.  of  surface 
per  cu.  ft. 

A  second  noteworthy  feature  of  soil  particles  is  their  power  to 
retain  certain  elements  that  are  brought  in  contact  with  them 
in  solutions.  These  elements  are  retained  in  some  instances 
much  after  the  manner  of  film  water  but  in  other  cases  retention 
is  effected  through  chemical  reactions.  You  see  an  illustration 
of  this  property  of  soils  when  colored  solutions,  such  as  the 
drainage  water  from  stables,  after  passing  through  a  soil  comes 
out  quite  colorless.  The  old  type  of  charcoal  filter  purified 
drinking  water  in  this  way.  It  is  an  important  fact  that  many 
of  the  important  elements  and  compounds  utilized  by  the  plant, 
such  as  potassium,  ammonium,  magnesium,  calcium,  phos- 
phorus, etc.,  are  retained  in  this  way;  and  they  are  retained  so 
firmly  that  only  small  percentages  of  them  are  leached  out  and 
lost  through  the  action  of  running  water.  Certain  aluminum 
compounds  (see  above),  also  iron,  calcium  and  the  humus  com- 
pounds derived  from  dead  plants  and  animals  are  very  important 
factors  in  retaining  the  above  mentioned  elements. 

21.  Relation  of  the  Root  Hairs  to  the  Soil  Particles. — We  are 
now  prepared  to  understand  the  significance  of  the  structures 
noted  in  the  root  "and  their  relation  to  the  physical  properties  of 
the  soil.  In  the  first  place  we  see  that  these  microscopic  root 
hairs  can  readily  penetrate  any  cavity  and  they  are  so  delicate 
and  soft  that  they  become  moulded  about  any  soil  particle  in 
their  line  of  growth  (Fig.  30,  B).  So  firmly  are  they  attached 
to  these  particles  that  it  is  impossible  to  detach  them  without 
injuring  the  root  hairs  (Fig.  29,  B).  This  arrangement  admir- 
ably adapts  them  to  gathering  up  the  water  and  other  crude 
material  in  the  soil.  The  tubes  are  filled  with  organic  acids  and 
other  substances  which  have  an  affinity  for  water  and  such  earth 


NATURE  OF   PLANTS  53 

substances  as  the  plant  requires.  In  this  way  the  materials  in 
the  soils  are  drawn  through  the  delicate  walls  of  the  root  hairs, 
a  process  termed  osmosis.  Some  of  the  acids  in  the  root  hairs 
may  also  reach  through  the  cell  walls  and  so  come  in  contact  with 
the  insoluble  substances,  dissolving  them  and  thus  rendering 
them  capable  of  absorption  by  osmosis.  Certain  it  is  that  CO2 
is  given  off  by  the  root  hairs  and  as  we  have  seen,  this  gas  enter- 
ing into  the  film  water  would  enable  it  to  dissolve  calcium — a 
fact  easily  demonstrated  by  allowing  the  roots  to  grow  over  the 
clean  surface  of  oyster  shells  or  pieces  of  polished  marble.  Deli- 
cate grooves  in  the  marble  show  where  the  roots  have  dissolved 
the  calcium  in  the  course  of  their  growth.  Some  roots  show  an 
acid  reaction,  as  is  indicated  by  their  ability  to  turn  blue  litmus 
paper  red ;  but  all  roots  do  not  react  in  this  way  and  it  is  impossible 
to  state  whether  acids,  other  than  carbonic  acid  which  would 
result  from  the  elimination  of  CO2,  are  excreted. 

We  are  now  prepared  to  understand  why  the  root  hairs  are 
developed  just  back  of  the  zone  of  elongation.  Any  disturbance 
of  these  delicate  absorbing  organs,  such  as  the  extension  of  that 
part  of  the  root  to  which  they  are  attached,  would  result  in 
tearing  and  killing  them. 

22.  Familiar  Facts  About  Roots. — The  character  of  the  root 
and  its  relation  to  the  physical  properties  of  the  soil  explain  many 
familiar  phenomena  of  plant  life.  We  see  why  plants  must  be 
repotted.  The  root  hairs,  which  only  live  for  a  few  days  or  a 
few  weeks,  are  constantly  being  formed  back  of  the  growing 
zone  and  dying  off  behind.  This  would  result  in  not  only  placing 
the  new  root  hairs  in  new  soil  but  they  would  also  be  removed 
in  a  measure  from  any  harmful  substances  that  may  have  arisen 
through  the  activity  of  the  older  root  hairs.  In  this  connection 
note  should  be  made  of  the  suggestive  work  that  is  being  ac- 
complished today  tending  to  show  that  roots  give  off  substances, 
probably  of  organic  nature,  which  are  toxic  in  character  and  so 
render  the  soils  unfit  for  plant  growth.  These  substances  are 
readily  oxidized  and  rendered  harmless.  The  presence  of  oxy- 
gen and  certain  fertilizers  increases  the  oxidizing  power  of  the 
roots.  While  this  theory  is  far  from  being  established  it  is  pos- 


54  RELATION   OF   ROOT  TO  SOIL 

sible  that  we  may  have  a  new  explanation  for  the  sterile  condition 
of  some  soils;  also  plowing  and  the  use  of  fertilizers  may  have  a 
new  meaning.  Consequently  when  a  potted  plant  shows  signs 
of  starvation  it  is  removed  from  the  jar,  the  tangled  mass  of  roots 
next  to  the  jar  is  cut  off  together  with  a  portion  of  the  earth  and 
then  repotted  with  fresh  earth.  In  this  way  room  is  given  for 
the  rootlets  to  grow  out  into  the  earth  and  develop  a  new  ab- 
sorbing surface  and  additional  materials  are  placed  at  the  disposal 
of  the  root.  This  relation  of  the  root  hairs  to  the  soil  particles 
explains  to  us  why  transplanted  seedlings  usually  wilt.  Many 
of  the  root  hairs  have  been  killed  and  the  seedling  does  not  revive 
until  new  root  hairs  have  been  developed.  So  also  it  is  clear 
why  most  shrubs  and  trees  should  be  transplanted  in  September 
or  October  or  in  the  early  spring  in  order  to  permit  the  rootlets 
which  renew  their  growth  very  early  in  the  spring  to  establish 
in  the  soil  new  absorbing  surfaces  before  the  leaves  develop  and 
bring  about  an  excessive  loss  of  water  through  transpiration. 
Why  are  plants  often  pruned  or  cut  back  in  transplanting? 
Why  is  nursery  stock  frequently  transplanted,  generally  a  year 
before  it  is  to  be  permanently  planted? 

You  have  often  noticed  that  fields  of  grain  and  other  crops  turn 
yellow  and  die  during  a  long  rainy  season.  This  is  due  to  the 
fact  that  the  air  spaces  between  the  earth  particles  become  filled 
with  water  and  there  is  no  longer  possible  an  interchange  of  gases. 
At  least  two-fifths  of  the  space  in  these  capillary  pores  should  be 
filled  with  air  to  insure  healthy  plants.  This  is  particularly 
noticeable  in  clay  soils  which  are  referred  to  as  cold,  wet,  and 
sour  soils.  The  reason  of  this  is  that  the  earth  particles  of  such 
soils  are  very  fine  and,  owing  to  their  enormous  surface,  they 
hold  the  water  with  great  tenacity.  In  this  way  acids  from 
decaying  vegetation  as  well  as  other  substances  accumulate  in 
excess  and  render  the  soil  unwholesome.  Therefore  heavy  clay 
soils,  which  are  richer  in  plant  foods  than  other  soils,  require  an 
admixture  of  sand  and  an  adequate  drainage  to  render  them 
sufficiently  porous  for  a  proper  circulation  of  air.  Sandy  soils 
are  made  of  coarser  particles  and  therefore  looser.  For  this 
reason  the  capillary  water  is  not  retained  for  any  considerable 


NATURE  OF   PLANTS    •  55 

time  and  the  soil  is  properly  aerated.  Injudicious  sprinkling  of 
lawns,  gardens,  or  potted  plants  will  result  in  the  formation  of  a 
compact  surface  crust.  This  acts  exactly  like  a  clay  soil  and 
draws  the  water  away  from  the  region  of  the  roots  to  the  surface 
of  the  soil  where  it  is  lost  by  evaporation.  Potted  plants  are 
best  watered  by  standing  the  jar  in  the  water  until  the  soil  is 
wet.  Further  watering  is  not  required  until  the  jar  sounds 
hollow  when  tapped.  So  gardens  and  lawns  should  be  thor- 
oughly soaked  during  the  night.  The  ordinary  sprinkling  only 
moistens  the  surface  and  leaves  the  soil  in  a  worse  condition  for 
holding  the  moisture  than  before,  because  on  drying  it  bakes 
and  the  hard  crust  draws  up  the  water  to  the  surface  where  the 
dry  air  readily  evaporates  it.  A  light  shower  in  the  summer  time 
may  produce  the  same  injurious  effect.  On  the  other  hand  when 
the  surface  of  the  soil  is  loose  and  air  dry,  so  that  the  capillary 
water  does  not  extend  from  the  surface  to  the  deeper  strata, 
then  a  shower  may  do  harm  in  quite  another  way.  By  supplying 
the  upper  particles  with  films  of  water  it  reestablishes  capillary 
connection  with  the  lower  lying  particles  so  that  the  film  water 
now  begins  to  move  again  towards  the  surface  of  the  soil.  This 
explains  why  the  top  soil  should  be  harrowed  and  hoed  during 
periods  of  inadequate  rainfall.  The  loosely  arranged  particles 
quickly  become  dry  and  the  capillary  films  leading  to  the  water 
supply  below  are  thus  broken  at  the  surface  and  further  loss  of 
water  through  evaporation  is  in  part  checked.  A  mulch  of  straw 
and  leaves  upon  the  surface  of  a  soil  has  the  same  result  as 
harrowing.  Sometimes  it  is  desirable  to  have  a  compact  surface 
layer  of  soil,  as  for  example  in  sowing  a  field  to  grain  or  grass. 
This  is  accomplished  by  rolling  the  field,  which  not  only  gives  a 
smooth  surface  to  facilitate  the  cutting  of  the  crop  but  also  makes 
a  compact  stratum  at  the  surface  which  draws  the  moisture  up 
to  the  grains,  causing  them  to  grow.  Otherwise  the  surface  of 
the  soil  containing  the  grains  would  be  so  dried  out  by  the  winds 
often  in  a  single  day,  as  to  prevent  their  sprouting. 

23.  Extent  of  the  Root  Surface. — It  may  seem  surprising  that 
the  roots  are  able  to  take  up  the  large  volumes  of  water  that  are 
lost  daily  through  transpiration.  The  spread  of  the  root,  how- 


56  ROOT   GROWTH 

ever  is  much  more  extensive  than  we  think,  usually  quite  equal 
to  that  of  the  branches,  so  that  the  absorbing  portion  of  the  root 
is  beneath  the  drip  of  the  branches  or  exposed  to  the  rains  and 
dews.  Furthermore,  when  we  come  to  add  together  the  lengths 
of  the  numerous  rootlets,  the  extent  of  the  total  root  system  is 
still  more  surprising.  The  total  length  of  the  roots  of  an  oat 
plant  has  been  estimated  at  154  feet;  of  a  corn  plant,  at  1320  feet; 
and  of  a  squash  plant  at  fifteen  miles.  These  roots  in  ordinary 
soil  reach  a  depth  of  from  three  to  five  feet,  but  in  dry  regions 
sometimes  much  greater  depths  are  attained ;  for  example,  alfalfa 
31  feet  and  mesquite  60  feet.  The  most  important  increase  in 
the  extent  of  the  root  system  is  due  to  the  root  hairs,  480  having 
been  estimated  on  .01  in.  of  a  root  of  corn  1/17  in.  in  diameter, 
and  230  on  a  square  mm.  of  a  pea  root.  By  this  means  the  ab- 
sorbing surface  of  roots  may  be  increased  from  five  to  ten  times. 
24.  The  Structure  of  the  Root. — You  have  noticed  that  only 
the  apical  portion  of  the  root  elongates  and  it  may  be  well  to  first 
examine  the  structure  of  this  region  in  order  to  see  how  growth 
is  brought  about  and  what  changes  are  effected  by  it.  The  tip 
of  the  root  is  composed  of  very  delicate  cells.  Certain  of  these 
cells  are  rapidly  dividing  by  forming  new  walls  through  the 
middle  of  the  cells,  thus  dividing  each  cell  into  two  new  cells. 
The  division  of  a  cell  is  effected  in  a  very  elaborate  manner.  The 
first  indication  of  this  growth  appears  in  the  nucleus.  This  organ 
of  the  cell  enlarges  (Fig.  31,  A)  and  we  now  see  that  it  has  a  very 
complex  structure,  consisting  of  a  protoplasmic  mass  surrounded 
by  a  delicate  membrane  and  traversed  by  a  network  of  delicate 
threads  (linin)  and  containing  one  or  more  large  granules  (nu- 
cleoli).  The  most  important  portions  of  the  nucleus  are  the 
minute  particles  (chromatin)  which  are  associated  with  the  linin 
threads.  It  has  been  supposed  that  this  substance  controls  to  a 
considerable  extent  the  activity  of  the  cell  and  determines  the 
kind  of  cell  that  shall  be  formed  and  the  work  that  it  shall  per- 
form. As  the  nucleus  enlarges  the  chromatin  increases  in  amount 
and  often  forms  a  ribbon-like  structure  (Fig.  31,  B)  which  finally 
contracts  and  divides,  thus  forming  numerous  small  masses  of 
chromatin,  called  chromosomes  (Fig.  31,  C).  At  this  stage  in  the 


NATURE   OF   PLANTS 


57 


division  delicate  colorless  strands  begin  to  appear  and  ultimately 
grow  into  a  spindle  as  shown  in  Fig.  31,  D.  There  are  two  sets  of 
these  strands  or  fibrillae;  an  inner  and  an  outer  series.  The  outer 
fibrillae,  in  some  way  not  understood,  arrange  the  chromosomes  in 
the  center  or  equator  of  the  spindle  (Fig.  31,  D)  where  each 


FIG.  31.  Cell  division  in  root  of  corn:  A,  cell  with  nucleus  enlarging  pre- 
liminary to  division — I,  linin;  ch,  chromatin.  The  central  dark  body  is  the 
nucleolus.  B,  later  stage,  the  chromatin  has  increased  and  appears  as  a  ribbon- 
like  skein.  C,  formation  of  the  chromosomes,  cr.  D,  formation  of  the  spindle 
and  the  arrangement  of  the  chromosomes  in  the  center  of  the  spindle. — 
I.  D.  Cardiff. 

chromosome  divides  by  a  longitudinal  division  into  two  equal 
parts.  The  fibrillae  now  separate  the  two  halves  of  each  chromo- 
some and  pull  them  to  the  opposite  poles  of  the  spindle  so  that 
each  pole  receives  a  half  of  every  chromosome  (Fig.  32,  E). 
The  chromosomes  now  become  rearranged  at  the  poles  and  two 
new  nuclei  are  gradually  formed  like  the  original  nucleus  (Fig. 
32,  F,  G).  In  the  meantime  the  inner  fibrillae  of  the  spindle  have 
shortened  and  become  thicker  at  the  equator  (Fig.  32,  G).  This 
thickening  goes  on  assisted  by  the  addition  of  new  fibrillae  at 
either  side  of  the  spindle  until  a  delicate  line  (the  cell  wall)  reaches 
across  the  old  cell  (Fig.  32,  H),  and  the  division  of  the  mother 
cell  into  two  daughter  cells  is  completed.  In  this  way  new  cells 
are  being  constantly  added  to  the  root.  It  will  be  noticed,  if  a 
longitudinal  section  through  the  middle  of  the  root  is  examined, 
that  various  regions  of  the  elongating  root  may  be  recognized 
owing  to  the  difference  in  the  character  of  their  growth.  In  such 
a  section  (Fig.  33)  we  see  that  the  tip  of  the  root  is  covered  with 
5 


58  STRUCTURE   OF  THE   ROOT 

a  mantle  of  cells,  the  root  cap.  This  cap  protects  the  delicate 
cells  within  like  a  thimble  so  that  they  are  not  exposed  or  injured 
as  the  root  extends  through  the  soil.  At  the  tip  of  the  root,  just 
within  the  root  cap,  the  cells  are  actively  dividing  and  adding 
new  cells  to  the  end  of  the  root  and  some  cells  are  also  added 


FIG.  32.  Later  stages  in  the  division  of  the  cell:  E,  the  fibrillae  pulling 
the  separated  halves  of  the  chromosomes  to  the  opposite  poles  of  the  spindle. 
The  inner  series  of  fibrillae  are  now  seen  at  the  equator  between  the  two 
groups  of  chromosomes.  F,  the  chromosomes  arranged  at  the  poles  and  the 
inner  fibrillae  increasing  in  size  and  number.  G,  the  fibrillae  have  increased 
in  number  until  they  nearly  reach  the  opposite  walls  of  the  mother  cell.  Their 
thickening  at  the  equator  is  the  first  indication  of  the  wall  separating  the  two 
new  cells.  H,  position  of  the  new  wall  clearly  indicated. — I.  D.  Cardiff. 

to  the  inner  side  of  the  root  cap  (Fig.  34).  Owing  to  this  unique 
arrangement  it  does  not  matter  if  the  outer  cells  of  the  cap  are 
injured  or  destroyed  as  the  root  pushes  through  the  soil,  because 
the  cap  is  constantly  being  renewed  from  within,  and  so  always 
furnishes  adequate  protection  to  the  delicate  cells  within.  The 
cells  that  are  added  to  the  tip  of  the  root  divide  several  times 
after  their  formation,  so  we  find  that  the  tip  of  the  root  for  a 
distance  of  one  or  two  mm.  is  composed  of  small  cells  that  are 
in  a  process  of  division  but  that  are  enlarging  to  only  a  slight 
degree  (Fig.  34).  This  region  is  called  for  this  reason  the  for- 
mative region  of  the  root.  Back  of  the  formative  region  for  a 
distance  of  two  or  four  mm.  the  cells  are  dividing  to  a  less  degree 
but  are  elongating  very  rapidly  and  changing  in  form  (Fig.  35). 
This  is  the  region  of  rapid  elongation.  Still  further  back  elonga- 
tion has  ceased,  but  the  walls  of  the  cells  are  becoming  thicker 


NATURE   OF   PLANTS 


59 


and  the  cells  are  changing  in  character,  so  that  they  perform 
different  duties.  This  is  illustrated  in  the  cross  section  of  the 
root  (Fig.  36),  where  we  see  that  the  outer  cells  have  become 
modified,  forming  an  epidermis  with  root  hairs 
for  absorption.  Within  the  epidermis  is  a  broad 
zone  of  cells,  the  cortex,  often  used  for  the  stor- 
age of  manufactured  foods,  while  in  the  center 
of  the  stem  are  the  vascular  bundles.  The  woody 
portion  of  the  bundle,  or  xylem,  radiates  out- 
ward from  the  center  and  the  soft  portion,  or 
phloem,  alternates  with  it  (Fig.  36,  x,  p).  The 
materials  absorbed  from  the  soil  are  largely 
transported  up  to  the  stem  and  leaves  through 
the  xylem,  and  the  foods  manufactured  by  the 
leaves  reach  the  root  by  means  of  the  phloem 
cells.  The  branches  of  the  root  originate  in  a 
very  curious  way  from  the  cells  just  outside  the 
xylem.  These  cells  by  repeated  divisions  form 
lateral  roots  which  gradually  destroy  the  tissues 
in  their  way  and  finally  grow  out  to  the  surface 
of  the  root  (Fig.  37).  By  this  arrangement  they 
are  provided  with  a  root  cap  and  fully  prepared 
to  enter  the  soil  on  emerging  from  the  old 
root. 

25.  The  Transport  of  Water  in  the  Root. — The  inner  layer  of 
the  cortex,  the  endodermis  (Fig.  36,  end),  consists  of  a  layer  of 
cells  which,  in  the  older  part  of  the  roots,  forms  a  very  compact 
and  more  or  less  cutinized  ring  of  cells.  This  tissue  is  supposed 
to  function  in  preventing  an  undue  loss  of  water  from  the  con- 
ducting strands  of  xylem  to  the  cortex.  It  should  be  stated  here 
that  the  root  hairs  not  only  absorb  fluids  but  also  force  out  the 
absorbed  substances  into  the  adjoining  cells  of  the  cortex.  All 
living  cells  have  this  power  of  absorbing  and  expressing  fluids. 
So  it  comes  about  that  the  absorbed  fluids  are  forced  by  the  root 
hairs  into  the  cortical  cells  and  by  them  they  are  passed  on  to  the 
cells  of  the  xylem  when  by  some  force  not  known  they  are  drawn 
up  the  stem  into  the  leaves.  The  force  exerted  by  these  myriad 


FIG.  33.  Dia- 
gram of  a  section 
taken  through 
the  center  of  a 
corn  root  :r,  root 
cap;  e,  epider- 
mis; c,  cortex;  p, 
central  region. 


6o 


ROOT  STRUCTURE 


FIG.  34.  Portion  of  the  formative  region  of  the  root  taken  at  a  in  Fig.  33: 
r,  root  cap;  v,  cells  which  by  repeated  division  add  new  cells  to  end  of  root 
and  to  inside  of  root  cap.  Note  the  numerous  dividing  cells  and  their  uni- 
formity in  size. — I.  D.  Cardiff. 

P  m 

e 


:. 


FIG.  35.  Portion  of  the  region  of  elongation  of  the  root  taken  at  b  in  Fig. 
33.  Note  the  larger  size  of  the  cells,  increase  in  cell  sap  as  indicated  by  the 
larger  vacuoles  and  absence  of  cell  division;  e,  epidermis;  c,  cortical  region; 
p,  central  region. — I.  D.  Cardiff. 


NATURE   OF   PLANTS  61 

number  of  cells  in  the  root  exerts  a  pressure  that  often  amounts 
to  considerably  more  than  one  atmosphere,  i.  e.,  15  Ibs.  to  the 
square  inch.  It  is  the  steady  expressing  of  fluids  by  these  cells 
that  causes  the  familiar  phenomena  of  the  "bleeding,"  or  flow 
of  water  from  stumps  in  the  spring;  likewise  the  "bleeding,"  of 
injured  branches,  or  the  flow  of  sap,  for  in  the  stem  the  living 


FIG.  36.  Cross-section  of  root  taken  above  section  shown  in  Fig.  33:  e, 
epidermis  with  root  hairs;  c,  cortex  bounded  on  inner  side  by  endodermis, 
end.  Within  is  the  central  region  containing  vascular  bundles;  x,  xylem;  />, 
phloem.—  I.  D.  Cardiff. 

cells  are  constantly  absorbing  and  giving  off  water  as  in  the  root. 
In  the  summer,  stumps  and  stems  do  not  "bleed"  as  a  rule  be- 
cause the  water  lost  by  transpiration  nearly  empties  the  cells, 
whereas  in  the  spring  before  the  leaves  appear  they  become  filled 
with  fluid. 

26.  The  Sensitiveness  of  the  Root. — We  may  now  ask  how 
does  this  elaborate  root  mechanism  become  so  perfectly  adjusted 
to  the  soil.  If  the  root  of  a  pea  or  bean  seedling  is  placed  hori- 
zontally in  sawdust,  after  one  or  two  hours  it  will  begin  to 
curve  down  toward  the  earth  center.  No  matter  in  what  position 
it  is  placed  the  result  is  always  the  same.  We  are  so  familiar 
with  the  downward  growth  of  roots  into  the  soil  that  we  never 


62  SENSITIVENESS   OF  THE   ROOT 

stop  to  consider  how  they  gain  their  sense  of  direction.  Gravity 
is  the  stimulus  that  acts  upon  the  irritable  protoplasm  of  the  cells 
and  so  brings  about  a  growth  reaction  that  sends  the  root  in  the 
right  direction.  When  roots  are  placed  in  a  horizontal  position 
and  slowly  revolved  on  their  longitudinal  axes,  no  curvature  re- 
sults, since  all  parts 'of  the  cells  are  stimulated  alike;  but  when 
allowed  to  rest  a  curvature  results  because  possibly  heavier  par- 
ticles in  the  cells  fall  to  the  lower  sides  of  the  cells  and  so  pro- 


FIG.  37.     Cross-section  of  a  root  of  lupine  showing  the  origin  of  the  lateral 
rootlets.     Lettering  as  in  Fig.  36. — H.  O.  Hanson. 

duce  an  irritation  through  their  unusual  position.  The  cells  in 
the  first  millimeter  and  a  half  of  the  root  tip,  or  possibly  in  the 
root  cap,  are  sensitive  principally  to  the  stimulus  of  gravity — 
the  other  cells  much  less  so.  If  this  region  of  the  root  is  care- 
fully removed  with  a  very  sharp  razor  the  root  is  no  longer 
capable  of  responding  to  gravity  although  it  may  curve  in 
various  directions  owing  to  the  irritation  produced  by  cutting. 
Furthermore,  while  the  tip  alone  perceives  the  stimulus  of 
gravity,  the  curvature  occurs  two  or  three  mm.  back  of  the 
tip,  i.  e.,  in  the  region  of  rapid  elongation,  so  that  we  have  the 
transmission  of  the  impulse  somewhat  after  the  manner  of  our 
own  nervous  system.  To  be  sure  there  are  no  specially  con- 


NATURE  OF  PLANTS  63 

structed  cells  for  conveying  this  impulse  comparable  to  the  nerves, 
but  it  has  been  shown  that  chemical  changes  are  set  up  that  ex- 
tend from  the  tip  to  the  region  of  curvature  where  a  more  ex- 
tended growth  of  the  cells  on  the  upper  side  of  the  root  is  induced 
than  on  the  lower  side.  This  results  in  bending  the  root  down 
into  the  soil.  These  reactions  apply  to  the  main  or  tap  root. 
The  lateral  branches,  or  secondary  roots,  tend  to  grow  more  or 
less  at  right  angles  to  the  stimulus  of  gravity,  while  the  short 
tertiary  branches  radiate  out  in  all  directions.  Why  the  stimulus 
of  gravity  causes  a  more  extended  growth  on  one  side  of  the  tap 
root  than  on  the  other  or  why  the  lateral  roots  are  so  differ- 
ently affected  can  not  yet  be  answered,  but  the  nicety  of  these 
reactions  in  enabling  the  roots  to  reach  in  all  directions  and 
thoroughly  explore  the  soil  in  their  quest  for  food,  is  very  ap- 
parent. The  cells  of  the  root  tip  are  also  sensitive  to  touch  or 
contact.  This  enables  the  root  to  avoid  obstacles  and  grow 
around  rocks  and  stones.  In  this  case  the  irritation  of  the  root 
tip  produces  a  more  considerable  growth  in  the  zone  of  curva- 
ture on  the  side  where  the  root  is  irritated  with  the  result  that 
the  tip  of  the  root  is  bent  from  the  object.  In  this  way  roots 
work  their  way  through  the  soil  and  avoid  obstacles  and  grow 
around  rocks,  for  as  soon  as  the  irritation  of  the  obstruction  is 
removed  by  the  curvature,  gravity  will  cause  the  root  to  grow 
down  again. 

The  cells  of  the  root  tip  also  react  to  light  intensities  and  to 
definite  percentages  of  moisture  and  mineral  foods.  Roots  avoid 
light  and  as  a  consequence  they  grow  down  into  the  earth  where 
they  are  directed  in  their  growth,  owing  to  the  peculiar  sensitive- 
ness of  their  cells,  so  that  they  come  to  lie  in  soils  containing 
suitable  moisture  and  crude  food  materials.  Every  one  is  fa- 
miliar with  the  fact  that  roots  are  caused  to  curve  towards  and 
follow  the  course  of  decaying  tree  trunks,  being  stimulated  by 
the  moisture  and  food  contained  in  them.  Numerous  instances 
might  be  cited  where  roots  are  attracted  to  wells,  drains,  etc., 
even  surmounting  considerable  obstructions  in  order  to  reach  the 
water.  This  localization  of  the  sensitiveness  to  all  external 
conditions  that  affect  the  root  at  the  very  tip  is  altogether  ad- 


64  ROOT  FUNCTIONS 

mirable,  because  it  enables  it  at  the  very  start  to  come  into  the 
most  helpful  relations  to  its  surroundings.  It  was  a  very  clever 
fancy  of  Darwin  to  compare  this  localization  to  a  brain  center 
as  found  in  certain  low  orders  of  animals. 

27.  Other  Root  Relations. — While  70  per  cent,  of  our  plants 
develop  roots  adapted  to  absorbing  substances  from  the  soil 
there  are  several  interesting  modifications  of  the  structures  noted 
above.  The  roots  of  aquatics  may  not  develop  root  hairs  be- 
cause the  roots  are  so  finely  divided  and  delicate  as  to  enable 
them  to  absorb  their  crude  materials  directly  from  the  water. 
Many  plants  develop  roots  in  the  air.  These  are  sometimes 
simply  clinging  devices,  as  in  the  poison  ivy;  or  they  may  be  true 
absorbing  organs,  in  some  cases  reaching  down  to  the  ground 
or  to  cup-shaped  leaves  that  hold  water,  as  in  many  tropical 
climbing  plants.  One  of  the  most  interesting  of  these  modifi- 
cations is  found  in  the  orchids  (Fig.  291)  that  live  on  the  damp 
tree  trunks  or  on  dripping  rocks  of  tropical  countries.  Here  the 
root  is  covered  by  a  thick  mantle  of  cells  that  are  capable  of  hold- 
ing the  water  that  comes  to  them  either  in  the  form  of  dew  or 
drainage  or  rain.  Such  plants,  called  epiphytes,  are  able  to 
flourish  in  the  air  without  the  assistance  of  soil  roots.  Quite  a 
large  number  of  plants,  parasites,  depend  entirely,  or  in  part,  upon 
other  plants  for  their  foods.  The  mistletoe  is  an  example  of  this. 
The  sticky  seeds  are  carried  by  birds  to  the  branches  of  oaks  and 
other  trees  where  they  germinate,  the  roots  penetrating  the 
branches  and  withdrawing  the  necessary  foods.  The  mistletoe 
withdraws  largely  crude  materials  from  the  branch  because  it 
has  pale  green  leaves  and  is  therefore  capable  of  manufacturing 
some  food  on  its  own  account.  Our  common  dodder  (Cuscuta) 
is  entirely  dependent  upon  other  plants  and  illustrates  the  results 
that  follow  from  the  disuse  of  any  function.  It  has  lost  its  chloro- 
plasts,  the  leaves  are  reduced  to  inconspicuous  scales  and  the  stem 
resembles  a  coarse  yellow  string  while  the  root  has  disappeared 
entirely.  The  seeds  produce  a  small  thread-like  shoot  which  soon 
perishes  unless  it  comes  in  contact  with  a  plant  upon  which  it 
can  live.  These  thread-like  plants  are  sensitive  to  touch  and  so 
coil  about  the  branches  of  any  plant  that  they  may  chance  to  hit 


NATURE  OF  PLANTS  65 

in  their  growth.  If  this  plant  happens  to  contain  foods  suitable 
to  the  dodder  then  a  second  stimulus  is  aroused  which  causes  root- 
like  branches  to  form.  These  organs  penetrate  the  branches  upon 
which  the  dodder  is  growing  and  absorb  foods  from  it.  Many 
plants  live  as  parasites  on  the  roots  of  a  variety  of  herbaceous  and 
woody  plants,  as  the  beech  drop,  orobanches,  broom  rape,  etc. 


FIG.  38.     Tubercles  formed  on  roots  of  lupine  by  nitrogen  fixing  bacteria. 

One  of  the  most  remarkable  features  of  roots  and  one  of  the 
most  important  in  the  economy  of  the  earth  is  seen  in  that  large 
family  of  bean  or  leguminous  plants  that  are  characterized  by 
flowers  and  fruit  like  those  of  the  pea.  Minute  plants,  bacteria, 
gain  access  to  the  roots  of  these  plants  through  the  root  hairs 
and  cause  wart-like  enlargements,  tubercles,  on  the  roots  (Fig. 
38).  These  bacteria  have  the  power  of  combining  the  nitrogen 
of  the  air  with  other  elements,  termed  the  fixation  of  nitrogen, 
and  so  forming  a  nitrogen-bearing  compound  that  can  be  ab- 
sorbed by  the  plant.  To  give  an  idea  of  the  importance  and 
work  of  these  bacteria,  it  is  estimated  that  an  acre  of  clover 
yielding  two  tons  contains  100  Ibs.  of  nitrogen — 74  Ibs.  of  which 
has  been  derived  from  the  air.  The  fixation  of  this  amount  of 
nitrogen  by  the  bacteria  in  the  roots  of  the  clover  would  nearly 
suffice  for  the  production  of  50  bushels  of  corn  and  30  bushels 
of  wheat.  Nitrogen  is  one  of  the  very  essential  elements  re- 


66  NITROGEN   FIXATION 

quired  in  the  construction  of  the  living  substance  of  the  plant. 
Strangely  enough,  though  the  atmosphere  contains  over  70  per 
cent,  of  this  gas,  the  plant  has  no  way  of  utilizing  it  directly  but 
only  in  compounds,  such  as  ammonia  and  nitrates,  etc.  Com- 
pounds of  this  kind  exist  in  very  meager  quantities  upon  the 
earth.  The  only  sources  of  supply  of  any  consequence  are  the 
decaying  animal  and  vegetable  life;  manures;  small  amounts 
that  are  formed  in  the  air  and  that  are  then  carried  to  the  earth 
in  rains;  the  guano  deposits,  now  practically  consumed;  and  the 
saltpeter  beds  of  Peru  and  Chile,  which  will  be  exhausted  by  1925. 
Consequently  widespread  alarm  has  arisen  lest  famines  ultimately 
result  through  lack  of  these  nitrogen  compounds,  and  the  situ- 
ation becomes  the  more  serious  since  from  one  half  to  two 
thirds  of  the  nitrogen  compounds  placed  on  the  soils  are  lost  an- 
nually in  various  ways,  especially  by  the  leaching  out  of  t?hese 
substances  by  rains  and  drainage  waters.  Naturally  of  late 
years  much  attention  has  been  directed  to  devices  for  uniting 
the  nitrogen  of  the  air  with  other  elements  in  order  to  find  a  sub- 
stitute for  the  rapidly  disappearing  nitrogenous  compounds. 
Working  upon  the  fact  that  nitrogen  compounds  are  formed  in 
the  atmosphere  by  electrical  discharges,  several  countries  are 
now  manufacturing  by  electrical  processes  large  quantities  of 
nitric  compounds.  One  of  the  valuable  substances  thus  formed 
is  calcium  nitrate.  Nitrogen  is  first  combined  with  oxygen  by 
utilizing  the  high  temperature  of  the  electric  arc  (2500  to  3000° 
C.)  and  the  nitric  oxide  gas  thus  formed  is  passed  through  milk 
of  lime,  thus  forming  calcium  nitrate.  Another  nitrogen  com- 
pound, valuable  for  certain  soils,  is  calcium  cyanimid.  It  is 
formed  by  passing  nitrogen  into  closed  retorts  containing  pow- 
dered calcium  carbide  heated  to  a  temperature  of  1100°  C. 
Under  this  temperature  the  calcium  carbide  (this  substance 
forms  acetylene  gas  when  placed  in  water)  unites  with  the 
nitrogen,  forming  calcium  cyanimid  and  carbon.  The  cyani- 
mid slowly  decomposes  in  the  soil,  yielding  ammonia  which  can 
be  either  directly  or  indirectly  utilized  by  the  plant.  While 
the  value  and  general  utility  of  these  artificially  formed  nitrogen 
compounds  has  not  been  thoroughly  tested,  it  is  not  to  be  ques- 


NATURE  OF  PLANTS  67 

tioned  that  products  and  processes  will  be  improved  so  as  to  meet 
the  rapidly  increasing  demands  for  nitrate. 

Much  study  has  also  been  given  to  the  nitrogen-fixing  bacteria 
as  a  further  source  of  supply  for  nitrogen  compounds.  Failure 
attended  the  numerous  attempts  to  cultivate  these  important 
plants  with  practical  results,  although  our  own  department  of 
agriculture  has  met  with  a  limited  success  in  making  cultures  of 
bacteria  that  are  sold  to  farmers  for  mixing  with  their  clover  or 
other  leguminous  seeds.  See  Bulletin  71,  1905,  U.  S.  Dept.  of 
Agriculture.  There  are  many  difficulties  in  the  way  of  intro- 
ducing these  forms,  such  as  keeping  them  alive,  supplying  the 
proper  conditions  for  their  growth  and  the  variety  of  forms  that 
must  be  dealt  with.  Although  there  is  possibly  but  one  species 
of  these  bacteria,  they  have  become  so  changed  through  associ- 
ation with  the  various  kinds  of  leguminous  plants  that  there  are 
numerous  varieties,  each  adapted  to  one  or  a  few  species  of  plants. 
Soils  in  one  section  may  have  the  forms  suitable  for  one  kind  of 
plant  and  be  totally  deficient  in  forms  required  for  other  plants. 
At  present  the  only  sure  way  of  inoculating  sterile  soils  is  by 
mixing  with  them  soils  containing  the  desired  bacteria — a  labori- 
ous and  expensive  method.  Quite  recently  very  promising 
results  have  been  obtained  by  growing  the  desired  bacteria  in 
soils  until  they  have  so  multiplied  and  permeated  it  that  a  small 
amount  of  this  soil  will  suffice  to  inoculate  a  large  area.  With- 
out doubt  means  will  be  devised  for  cultivating  the  various  forms 
of  these  bacteria  so  that  they  may  be  sold  for  mixing  with  the 
soil  or  seeds.  Thus  the  farmer  will  be  able  to  raise  a  crop  that 
contains  a  large  amount  of  nitrogen  derived  from  the  air  and  by 
plowing  under  this  crop  or  better  still,  if  conditions  permit,  by 
feeding  it  to  stock  and  utilizing  the  manure,  he  will  be  able  to 
add  a  valuable  nitrogenous  fertilizer  to  the  soil.  In  this  way  the 
soil  is  prepared  for  another  crop,  as  wheat,  which  is  dependent 
for  nitrogen  upon  nitrogenous  compounds.  This  alternation  of 
crops  is  of  prime  importance  in  successful  farming.  Each  kind  of 
plant  takes  from  the  soil  certain  elements  in  large  amounts  and 
for  this  reason  the  alternating  or  succeeding  crop  should  be  of 
such  a  kind  as  to  require  other  elements.  As  these  elements  are 


68  NATURE   OF   MYCORRHIZA 

removed  from  the  soil  suitable  fertilizers  containing  nitrogen, 
phosphorus,  lime,  potash,  etc.,  must  be  added  to  the  soil  to 
replace  the  absorbed  crude  foods. 

The  majority  of  plants  do  not  absorb  materials  from  the  soil 
in  so  direct  a  way  as  is  illustrated  in  Fig.  29.  They  are  de- 
pendent for  certain  of  their  materials  at  least  upon  a  low  group 
of  plants  termed  fungi.  These  fungi  more  commonly  consist  of 
delicate  cobwebby  threads  (termed  hyphae),  such  as  are  seen 
on  mouldy  bread.  This  fungal  group,  known  collectively  as 
mycorrhiza,  comprises  a  number  of  forms  or  species,  only  a  few 
of  which  have  as  yet  been  identified.  These  delicate  plants 
spread  through  the  soil  and  their  hyphae  enter  either  the  outer 
cells  of  the  roots  or  form  a  mantle  of  more  or  less  closely  inter- 
woven threads  about  them  or  the  hyphae  may  sustain  both  of 
these  relations  to  the  root.  It  has  been  demonstrated  in  many 
instances  that  they  transfer  to  the  plant  various  substances 
derived  from  the  humus.  Certain  forms,  probably  a  much  larger 
number  than  is  now  known,  have  the  power  to  fix  free  nitrogen 
much  after  the  manner  of  the  bacteria.  It  is  also  probable  that 
they  sometimes  take  over  the  normal  absorbing  function  of  th*e 
root  hairs  for  frequently  roots  associated  with  Mycorrhiza  develop 
root  hairs  sparingly  or  not  at  all.  This  relationship  is  not 
always  one  sided  in  its  benefits  for  the  fungus  receives  carbo- 
hydrate material  from  the  green  plant.  This  state  where  two 
or  more  plants  live  together  is  termed  symbiosis.  All  stages  in 
the  symbiotic  relationship  of  the  fungi  and  the  green  plants  may 
be  seen.  The  green  plant  and  the  fungus  may  not  be  benefited  at 
all,  a  relationship  termed  commensalism.  When  the  green  plant 
is  injured  by  the  fungus,  this  symbiotic  relationship  is  termed  para- 
sitism. The  symbionts  may  be  equally  benefited  and  not  at  all  in- 
jurious one  to  the  other.  The  majority  of  our  poplars,  willows, 
beeches,  heaths,  orchids  and  evergreens  have  attained  this  state 
and  do  not  flourish  in  soils  where  suitable  fungi  do  not  abound. 
Perhaps  this  explains  why  it  is  so  difficult  to  transplant  certain 
shrubs  and  trees.  The  fungi  are  easily  injured  and  do  not  be- 
come established  in  the  new  soil  soon  enough  to  keep  the  plant 
alive.  This  mutual  relationship  has  gone  so  far  in  some  species 


NATURE   OF   PLANTS  69 

that  the  seeds  are  unable  to  grow  unless  they  come  in  contact 
with  the  hyphae  of  the  fungus.  Finally  some  plants  have  become 
so  adapted  to  these  fungi  as  to  receive  all  necessary  foods  from 
them.  Accordingly,  their  green  leaves  and  roots  have  largely 
disappeared  since  they  are  no  longer  of  service.  Examples  of 
this  are  seen  in  the  white  Indian  pipe  and  pine-sap  and4n  the 
coral  root  orchid.  In  these  latter  examples  the  relationship  of 
the  two  plants  is  completely  turned  about  for  we  see  that  the 
plant  has  become  a  parasite  upon  the  fungus. 

28.  Roots  as  Store-houses  for  Foods. — In  all  the  cases  hereto- 
fore considered  the  root  has  functioned  in  one  way  or  another  as 
an  organ  for  the  temporary  reception  and  transmission  of  materi- 
als from  the  soil.     It  may  also  serve  in  other  capacities,  one  of 
the  more  important  of  which  is  as  a  storage  organ.     Such  roots 
become  fleshy  and  filled  with  foods  and  are  of  great  economic 
importance,  furnishing  a  variety  of  nutritious  vegetables,  as  the 
sweet  potato,  beet,  turnip,  etc.     Many  of  these  valuable  plants 
are  biennials.     During  the  first  season  the  plant  develops  only 
leaves  and  stores  up  food  in  its  fleshy  roots  which  is  utilized  in 
the  following  season  in  the  production  of  flowers  and  seeds,  after 
which  the  plant  perishes.     It  is  noteworthy  that  the  biennial 
habit  has  been  induced  in  many  of  these  plants  by  cultivation.     If 
they  are  left  to  themselves  in  a  wild  condition  they  will  soon 
revert  to  an  annual  growth,  i.  e.,  producing  seed  and  perishing 
during  the  season.     Perennial  plants  live  on  from  year  to  year. 
As  to  whether  some  plants  behave  as  annuals,   biennials,   or 
perennials  depends  in  some  cases  upon  the  place  or  time  of 
planting.     The  castor  bean  with  us  is  an  annual  but  in  warm 
countries  it  becomes  a  perennial.     Winter  wheat  on  the  other 
hand  has  acquired  a  biennial  habit  owing  to  the  late  planting  of 
summer  wheat  types. 

29.  Anchoring  and  Supporting  Roots. — Roots  also  play  a  very 
important  role  in  anchoring  and  supporting  the  plant.     Excellent 
examples  of  the  supporting  roots  are  seen  in  the  Indian  coyn 
where  numerous  roots  spring  from  the  stem  a  short  distance  from 
the  ground  and  reach  out  on  all  sides  like  guy  ropes  steadying 
the  plant  in  the  ground.     Similar  devices  appear  in  our  elms, 


70  BINDING  ACTION  OF  ROOTS 

maples,  and  beeches  where  enlargements  of  the  roots  at  the  base 
of  the  trunk  rise  up  like  girders,  bracing  the  tree  against  winds 
More  marked  illustrations  appear  in  many  tropical  plants,  as  the 
stilt  roots  of  the  mangrove  and  screw  pines,  and  the  buttressing 
roots  of  one  of  the  Indian  rubber  plants  (Ficus),  etc.  In  the 
banyan  tree  of  India  the  branches  have  an  almost  unlimited 
lateral  growth  owing  to  the  fact  that  they  are  supported  by  a 
succession  of  roots  that  reach  from  the  branches  to  the  ground. 
Kerner  cites  an  example  of  one  of  these  trees  with  300  large  and 
3,000  small  prop  roots.  This  tree  sheltered  a  village  of  100  native 
huts  and  an  army  of  5,000  men. 

30.  Binding  Action  of  Roots. — The  landscape  would  be  con- 
stantly subject  to  great  change  either  by  erosion  or  through  the 
action  of  winds  were  it  not  for  the  solidifying  and  binding  action 
of  roots  upon  the  soil.  The  difficulty  of  breaking  up  the  prairies 
and  the  turf  of  abandoned  fields  or  meadows  indicates  the  extent 
and  completeness  of  the  ramifications  and  interweaving  of  the 
roots.  Miles  of  sandy  reaches  are  held  from  shifting  through 
the  restraining  action  of  roots  and  stems  of  grasses  and  other 
plants.  The  little  town  at  the  end  of  Cape  Cod  would  have  been 
submerged  long  ago  by  the  shifting  sand  dunes  had  not  suitable 
plants  been  planted  to  hold  in  check  the  loose  sands. 

Roots  not  only  bind  the  soil  together  but  the  older  portion  of 
the  root  usually  possesses  the  power  of  contraction.  This  pro- 
perty results  in  the  pulling  down  and  fixing  of  the  stem  in  the 
ground.  You  must  have  often  wondered  how  stems  and  bulbs 
become  so  deeply  buried  in  the  soil  although  the  seeds  are 
scattered  on  the  surface  of  the  soil.  This  is  also  well  illustrated 
in  the  tips  of  raspberry  bushes  which  come  in  contact  with  the 
soil  through  the  bending  of  the  stalk.  Roots  strike  out  from  the 
tip  and  when  thoroughly  established  in  the  earth  the  older 
portions  contract  and  bury  the  tip  in  the  soil.  In  the  same 
way  seedlings  of  various  plants  are  slowly  pulled  down  into  the 
earth  so  that  they  finally  become  planted  at  considerable  depths. 
It  is  this  contraction  of  the  roots  that  keeps  the  slowly  elongating 
stems  of  the  dandelion,  dock  and  many  other  plants  buried  in 
the  soil  and  that  binds  many  creeping  plants  such  as  certain 
clovers  and  knot  weeds  firmly  to  the  earth. 


CHAPTER   III 


THE  STEM 


31.  The  Function  of  the  Stem. — The  stem  has  for  its  chief 
function  the  production  and  display  of  the  leaves  and  roots  and 
the  conduction  of  the  materials  which  these  organs  are  especially 
concerned  in  handling.     It  serves  as  a  connection  between  them, 
carrying  up  the  material  absorbed  by  the  roots  and  distributing 
the  various  substances  received  from  the  leaf.     Like  the  traffic 
of  a  city  this  material  is  received  at  many  stations  and  trans- 
ported along  various  channels  to  many  points.     At  one  place 
some  of  it  is  used  in  the  manufacture  of  food,  at  another  point 
material  is  required  for  the  nourishment  and  construction  of  the 
cells.     Here  a  portion  is  stored,  or  again,  there  is  the  useless 
or  waste  material  to  be  carried  away. 

32.  Character  of  the  Stem. — In  order  to  comprehend  the  nature 
of  the  work  performed  by  the  stem  it  will  be  necessary  to  ex- 
amine its  external  form  and  its  internal  structure.     While  stems 
vary  greatly  in  character  we  may  gain  a  general  understanding 
of  their  more  important  features  by  studying  some  woody  twigs 
or  branches  as  they  appear  in  winter  (Fig.  39) .     The  most  strik- 
ing features  in  such  twigs  are  the  buds,  'the  leaf  scars  (see  page  43) , 
lines  of  fine  scars  forming  rings  about  the  stems  at  varying  inter- 
val?, minute  roundish  or  lense-like  elevations  (the  lenticels),  and 
finally  the  character  of  the  branching.     The  buds  of  the  hickory, 
horse  chestnut,  Norway  maple,  lilac,  cherry,  etc.,  are  excellent 
for  study.     In  these  buds,  as  in  most  examples,  the  outer  part 
consists  of  a  series  of  leathery,  closely  overlapping  scales  or 
modified  leaves.     These  function  to  protect  the  delicate  parts 
within  against  injury  and  especially  against  loss  of  water.     By 
carefully  removing  the  scales  with  a  sharp  pen  knife  we  find 
within  the  ordinary  green  leaves,  stem  and  also  in  some  buds 
the  flowers  that  will  appear  next  year.     These  organs  are  so 
small  and  crowded  that  they  appear  to  arise  from  a  single  point. 


72  CHARACTER  OF  BUDS 

They  have,  however,  the  same  arrangement  as  in  the  mature 
shoot  of  the  summer  time  and  you  should  observe  for  a  few  days 
in  the  spring  the  opening  of  a  variety  of  buds,  noting  how  this 
crowded  arrangement  of  the  organs  of  the  bud  gives  place  to 
that  of  the  mature  shoot.  The  formation  of  buds  possibly  came 
about  owing  to  climatic  changes.  The  earlier  vegetation  of  the 
earth  was  subject  to  a  uniform  tropical  climate  but  the  formation 
of  high  mountain  ranges  and  other  factors  caused  cold  currents 
of  air  to  sweep  over  the  earth  and  also  produced  variations  in  the 
humidity  of  the  atmosphere.  These  factors  resulted  in  producing 
a  season  favorable  for  growth  and  one  in  which  growth  would 
be  stunted  or  checked.  The  latter  condition  induced  many 
changes  in  the  plant,  of  which  the  bud  is  an  important  example. 
The  great  majority  of  woody  plants  are  characterized  by  these 
two  phases:  a  period  of  rapid  growth  up  to  about  the  middle 
of  July,  during  which  time  all  the  organs  of  the  season  as  well  as 
the  buds  are  formed,  after  which  they  continue  to  rhanufacture 
food  and  store  it  up  in  the  buds  and  other  organs  until  September 
or  October;  and  a  period  of  dormancy  in  which  the  plant  is  in  a 
resting  condition  until  the  spring.  Such  a  method  of  growth  is 
termed  definite  and  should  be  distinguished  from  the  indefinite 
growth  of  a  few  plants  such  as  the  raspberries,  locusts,  some 
honeysuckles,  etc.,  where  the  growth  goes  on  until  the  fall  frosts, 
with  the  result  that  the  more  recently  formed,  delicate  parts  are 
killed  off  each  year.  But  even  in  these  cases  we  have  essentially 
the  same  rhythm  of  growth  as  in  the  first  case  because  the  older 
portions  of  the  shoot  have  perfected  their  tissues  and  developed 
their  buds  and  so  prepared  for  the  dormant  period. 

Whatever  may  be  the  causes  that  have  resulted  in  the  forma- 
tion of  the  bud,  certainly  its  closely  united  scales  which  are  often 
reinforced  with  resinous,  mucilaginous  or  hairy  coatings,  are  an 
admirable  device  forprotecting  the  delicate  parts  within  against 
winds  which  would  rob  them  of  water  at  a  time  when  none  could 
be  obtained  from  the  soil.  It  is  quite  natural  to  think  of  these 
devices  as  protection  against  cold,  but  strangely  there  is  no  adap- 
tation known  among  plants  that  serves  primarily  as  a  protection 
against  low  temperatures,  and  there  is  no  cold  upon  the  earth 


NATURE   OF   PLANTS 


73 


B 


FIG.  39.  Types  of  stems:  A,  branch  of  hickory  showing  a  large  terminal 
bud  and  several  small  lateral  buds  that  were  developed  in  the  axils  of  the 
leaves  of  the  past  season.  The  branch  is  three  years  old,  as  shown"  by  the 
three  rings,  r,  rr,  r",  of  the  bud  scars  which  mark  the  successive  positions  of 
the  terminal  bud  during  the  past  three  seasons;  /,  leaf  scars.  B,  branch  of 
pear — after  Bailey.  The  ring  at  I  shows  the  position  of  a  bud  which  produced 
the  next  year  a  pear,  as  is  indicated  by  the  large  scar  and  swollen  branch  at  a. 
A  short  lateral  branch  was  also  developed  the  same  season  with  its  terminal 
bud  at  2.  The  next  year  this  bud  produced  a  branch  that  extended  to  3, 
bearing  several  leaves  (note  leaf  scars)  and  of  course  axillary  buds — one  of 
which,  6,  we  see  grew  in  the  following  seasons.  No  fruit  was  developed  this 
season.  The  bud  formed  at  3  behaved  the  next  season  very  much  as  the  one 
previously  noted  at  I,  forming  a  pear  at  a'  and  a  short  lateral  branch  that 
reached  to  4.  Note  also  that  the  axillary  bud,  b,  of  the  previous  season  grew 
a  little  and  its  subsequent  history  can  be  followed  by  the  annual  bud  scars. 
In  the  following  season  bud  4  developed  a  vigorous  shoot  reaching  to  5.  No 
fruit  was  formed  but  three  buds  survived,  b',  b",  b'",  thus  nearly  duplicating 
the  growth  of  bud  2.  Bud  5  develops  the  next  season  fruit  at  a"  and  also 
two  lateral  shoots  that  extend  to  6  and  7  respectively.  The  three  buds  below 
5  had  only  a  feeble  growth  during  this  and  the  succeeding  seasons  and  bud  b"' 
evidently  perished  during  its  first  period  of  growth.  Bud  6  produces  the  next 
year  a  pear  at  a'"  and  a  lateral  shoot  reaching  to  8  and  bud  7  at  the  same 

6 


J 


74  NATURE  OF  BUDS 

so  intense  that  it  may  not  be  endured  by  many  plants,  if  this 
were  the  only  unfavorable  factor. 

Buds  are  usually  found  singly  in  the  axils  of  the  leaves  and  as 
a  rule  only  a  few  that  are  favorably  situated  as  regards  the  light 
ever  develop.  The  development  of  the  buds  brings  about  the 
characteristic  appearance  or  habit  of  the  plant.  This  growth  is 
not  controlled  alone  by  the  favorable  position  of  the  buds  but 
also  by  the  interaction  or  correlation  of  all  the  growing  parts. 
As  a  result  in  some  trees  we  find  a  single  terminal  bud  that  is 
larger  and  better  developed  than  all  others.  This  bud  will  pro- 
duce the  longest  shoots  and  consequently  such  trees  will  have 
spire-like  stems  as  in  the  spruces,  larches,  etc.  In  other  cases 
this  same  type  of  growth  may  continue  for  a  time  but  eventually 
terminal  buds  of  equal  vigor  will  be  developed  upon  several 
branches  and  consequently  equal  growths  or  diffuse  types  of 
branching  will  result  as  in  the  maples,  elms,  etc.  Buds  that  are 
not  favorably  located  generally  perish  after  a  few  years  but  not 
infrequently  they  remain  alive  and  become  overgrown  by  the 
increase  of  the  stem.  In  such  cases  the  bud  grows  slowly  and 
maintains  itself  near  the  surface  of  the  wood.  These  buds  grow- 
ing and  often  branching  in  the  wood  produce  those  curlings  and 
twistings  in  the  grain  that  are  commonly  known  as  bird's-eye 
wood.  Somewhat  similar  markings  are  also  produced  by  minute 
outgrowths  on  the  surface  of  the  wood.  These  do  not  have, 
however,  the  darkened  center  characteristic  of  the  curl  caused 
by  the  bud  growth.  Dormant  buds  sometimes  develop  into 
normal  branches,  as  when  a  portion  of  a  tree  is  removed  and 

time  only  develops  a  leafy  shoot  reaching  to  9.  Buds  8  and  9  both  produce 
fruit  the  next  season  (aiv,  avi}  and  short  lateral  branches  reaching  to  10  and  n. 
These  buds  during  the  present  season  only  formed  leafy  shoots. 

Buds  are  formed  in  the  axils  of  all  the  leaves  that  appear  on  the  shoots  of  a 
season's  growth,  but  notice  that  but  few  or  none  at  all  develop.  Some  have 
a  thrifty  growth,  others  remain  small  and  perish  after  one  or  more  years. 
This  is  true  of  the  fruit.  Some  of  the  pears  mature,  as  is  indicated  by  size 
of  the  scars,  while  others  drop  off  after  one  or  more  months.  Note  also  the 
almost  regular  alternation  in  the  production  of  shoots  bearing  fruit  and  leaf 
shoots.  Bud  6  is  the  only  exception  to  this  succession.  Observe  that  the 
branches  assume  different  positions  and  that  the  extent  of  the  elongation  of 
the  shoot  from  year  to  year  varies.  Can  you  explain  these  facts? 


NATURE   OF   PLANTS  75 

consequently  an  additional  supply  of  food  stimulates  them  to 
growth.  Buds  may  also  arise  upon  any  part  of  the  stem,  root 
or  leaf.  These  are  the  so-called  adventitious  buds  and  their 
formation  upon  roots  often  accounts  for  the  colonial  habit  of 
many  trees,  plants  and  shrubs,  as  in  the  poplars,  Ailanthus, 
sumacs,  etc.  They  appear  to  rise  naturally  in  some  cases,  but 
in  other  instances  their  formation  is  due  to  the  stimulus  of  a 
wound  or  some  other  cause  and  like  the  dormant  bud  they  serve 
to  prolong  the  life  of  the  plant.  Richards  has  shown  when 
living  cells  are  exposed  to  the  oxygen  of  the  air,  owing  to  a 
wound,  that  these  cells  are  stimulated  as  a  result  to  renewed 
activity.  This  doubtless  explains  the  formation  of  callus  and 
the  healing  of  wounds,  as  well  as  the  formation  of  adventitious 
buds  in  many  cases.  Common  examples  of  buds  due  to  wounds 
are  seen  in  the  vigorous  shoots  that  spring  up  from  the  stumps 
of  hardwood  trees,  pollarded  willows,  etc.  Some  buds  become 
fleshy  owing  to  the  storage  of  the  food  and  dropping  from  the 
plants  serve  to  propagate  new  individuals.  Examples  of  this 
are  seen  in  the  fleshy  buds  on  the  tips  of  the  branches  or  in  the 
axils  of  the  leaves  of  the  stone-crop  and  some  lilies  and  in  the 
flower  clusters  of  some  onions.  Many  aquatic  plants  have  the 
habit  of  forming  similar  buds  on  the  approach  of  winter.  These 
being  compact  and  heavy  with  food  sink  to  the  bottom  of  the 
ponds  in  the  fall  and  renew  their  growth  in  the  spring. 

The  rapid  unfolding  of  the  bud  in  the  spring  is  a  constant 
source  of  surprise  but  when  we  recall  that  it  contains  usually 
an  abundant  supply  of  food  and  practically  all  the  organs  that 
will  appear  on  the  stem  during  the  season  we  can  understand 
how  the  bud  opens  and  elongates  into  a  shoot  bearing  leaves 
and  flowers  during  the  first  few  weeks  of  spring.  The  part  of 
the  stem  bearing  the  scale  leaves  of  the  bud  does  not  elongate 
materially  since  these  leaves  are  of  service  only  during  the  winter 
or  dry  season  and  soon  fall  off  after  the  opening  of  the  bud. 
Consequently  the  scars  formed  by  the  fall  of  these  protective 
scales  form  a  ring  (Fig.  39,  r)  each  year  about  the  stem  which 
marks  the  position  of  each  successive  bud.  These  scars  are 
therefore  known  as  annual  rings  and  by  counting  the  number  of 


76  REGIONS   OF  THE  STEM 

these  rings  on  a  twig  you  can  ascertain  its  age  and  observe  the 
extent  of  elongation  during  each  season.  The  minute  dots  or 
lenticels  consist  of  loosely  arranged  cells  that  permit  an  inter- 
change of  gases  between  the  living  cells  within  the  stem  and  the 
atmosphere,  functioning  in  much  the  same  way  as  do  the  stomata 
of  the  leaves. 

We  are  now  interested  to  learn  something  of  the  internal  char- 
acter of  the  stem.  By  cutting  with  a  sharp  knife  across  the  stem 
between  the  terminal  bud  and  the  first  annual  ring  we  see  that 
there  are  several  regions  in  the  stem.  On  the  outside  is  a  very 
thin  brown  layer,  largely  composed  of  cork  cells,  which  prevents 
loss  of  water  and  which  in  old  stems  becomes  quite  thick  and 
variously  split.  Within  is  a  zone  of  rather  delicate  tissue.  The 
outer  part  of  this  region,  termed  the  cortex  contains  more  or  less 
chlorenchyma — at  least  this  is  true  of  small  stems — and  functions 
as  in  the  leaf;  while  the  inner  rather  colorless  portion  of  this 
region  serves  to  conduct  foods  manufactured  by  the  leaves  and 
it  also  frequently  serves  as  a  storehouse.  The  third  zone  is  made 
up  of  thick-walled,  compact  cells,  regularly  arranged.  This  is 
the  wood  or  xylem  portion  of  the  stem.  It  gives  stability,  con- 
ducts the  water  and  crude  materials  absorbed  from  the  soil  and 
also  serves  as  a  storage  organ  for  the  reserve  foods  of  the  plant. 
The  center  of  the  stem,  known  as  the  pith,  consists  of  delicate 
cells  which  conduct  water  for  a  short  time  and  soon  die. 

33.  The  Anatomy  of  the  Stem. — We  are  now  desirous  of  seeing 
how  this  complex  structure  of  the  stem  comes  about  and  of 
studying  in  more  detail  the  character  and  operations  of  the 
apparatus. 

If  thin  sections  are  made  across  the  stems  of  seedlings,  as  of 
the  castor  bean,  the  tissues  will  resemble  the  arrangement  shown 
in  Fig.  40.  It  should  be  stated  that  the  vascular  bundles  (Fig. 
40,  v)  do  not  appear  as  separate  strands  in  the  earliest  stages  of 
the  development  of  the  stem.  This  vascular  tissue  appears 
rather  as  a  more  or  less  continuous  circle  of  tissue  surrounding 
the  pith.  Very  early  in  the  development  of  the  stem  this  circle 
of  vascular  tissue  is  broken  into  segments  or  vascular  bundles. 
This  separation  of  the  ring  of  vascular  tissue  into  segments  is 


NATURE   OF   PLANTS 


77 


caused  by  strands  of  it  here  and  there  extending  out  into  the 
leaves,  thus  forming  gaps  in  the  circle.     All  seedling  stems  show 
this   arrangement   of   tissues   and   consequently   there   appears 
more  or  less  clearly  three  regions;  an  epidermis,  a  cortex,  and  a 
central  region  surrounded  by  the  cortex,  an  arrangement  already  , 
noticed  in  the  roots.     The  epidermis  does  not  materially  differ  in  I 
structure,  modification,  or  function  from  that  already  noted  in  the 
leaf,  see  page  8.     The  cortex  is  largely  composed  of  parenchyma 

*  *  •""~~""~~  —  ^7  ^"XN| 

and  extends  from  the  epidermis  to  the  endodermis  which  it  in- 
cludes. The  endodermis  is  not  so  well  marked  as  in  the  root 
and  owing  to  the  rapid  growth  of  the  tissues  in  this  region  it  is 


FIG.  40.  Cross-section  of  young  stem  of  castor  bean:  e,  epidermis;  c,  cortex  ; 
p,  pith  or  inner  portion  of  central  cylinder;  v,  vascular  bundle,  arranged  in 
outer  part  of  central  cylinder. — H.  O.  Hanson. 

often  impossible  to  detect  it.  The  cortex  assists  the  leaves  in 
photosynthesis,  the  outer  portion  as  a  rule  being  well  supplied 
with  chloroplasts.  In  old  stems  that  become  covered  with  a 
thick  layer  of  bark  the  chlorophyll  quite  disappears,  owing 
doubtless  to  its  exclusion  from  air  and  light.  The  cortex  also 
serves  as  a  storehouse  for  foods.  Particularly  is  this  true  of  the 
endodermis  and  adjoining  cells  which  are  often  temporary 


78  STEM   STRUCTURE 

receiving  stations  for  the  carbohydrates  during  their  transport 
through  the  stem.  The  cells  in  the  outer  portion  of  the  cortex 
frequently  become  thickened  and  more  or  less  elongated  to  give 
strength  to  the  stem.  One  of  the  most  common  modifications  of 
this  kind  is  shown  in  Fig.  41,  A.  This  tissue,  collenchyma,  is 
characterized  by  the  thickening  of  the  cells  at  the  angles  or  on 
all  sides  and  by  the  silvery  luster  of  the  walls.  The  walls, 
though  very  elastic  and  tough,  are  capable  of  growth  and  so 
they  are  especially  adapted  to  the  support  of  the  young  elongating 
stems.  In  older  stems,  where  elongation  has  ceased,  other 
greatly  elongated  cells,  called  stereome  fibers  or  sclerenchyma 


FIG.  41.  The  stereome  or  strengthening  cells  of  the  cortex:  A,  collenchyma 
cells  in  growing  stem  of  Begonia,  showing  above  the  cells  in  cross-section  and 
below  in  longitudinal  section.  B,  a  group  of  elongated  thick-walled  cells, 
called  sclerenchyma  fibers  or  stereome  fibers,  from  the  stereome  of  mature 
flax  stem. 

fibers,  are  often  formed.  These  cells  have  thick  walls  and  taper- 
ing ends  which  interlock  and  bind  the  cells  very  firmly  together 
(Fig.  41,  B).  These  strengthening  cells  form  a  compact  zone 
about  the  stem  or  they  may  be  arranged  in  separate  bundles. 
In  this  latter  case  they  often  appear  to  the  eye  as  light  colored 
bands  extending  along  the  surface  of  the  stems  of  many  herba- 
ceous plants.  The  central  region  of  the  stem  is  characterized 
by  a  mass  of  parenchyma  with  several  vascular  bundles  arranged 
in  a  circle  (Fig.  40).  The  central  mass  of  parenchyma  is  called 
the  pith. 

34.  The  Vascular  Bundle. — The  vascular  bundle  contains  three 
distinct  regions;  an  inner  thick-walled  portion  (the  wood  or  xy- 


NATURE   OF   PLANTS      ^  79 


lem),  an  outer  thin-walled  portion  (phloem),  and  between  these 
two  regions  a  delicate  layer  of  cells,  the  cambium  (Fig.  42) 
The  transport  of  all  substances  is  largely  c^nEned^to  the  vas- 
cular bundles,  the  xylem  conducting  principally  the  crude  rAa^ 
terials  while  the  bulk  of  the  organic  substances  passes  through 
the  phloem.     We  are  now  interested  to  study  the  character  o 
these  cells  and  note  their  adaptation  to  the  work  in  hand.     In  I 
the  xylem  occur  various  large  spaces,  the  vessels  or  ducts  (Fig. 
42,  z>),  and  smaller  spaces,  wood  cells  of  different  kinds.     The 


FIG.  42.  One  of  the  vascular  bundles  shown  in  Fig.  40  enlarged:  x,  xylem; 
v,  vessels  or  ducts;  p,  phloem;  s,  sieve  tube;  ac,  accompanying  cell;  c,  cam- 
bium; st,  stereome. — H.  O.  Hanson. 

cells  of  the  phloem  are  much  smaller,  thinner  walled  and  less 
numerous  than  those  of  the  xylem.  Therefore  very  thin  sec- 
tions are  necessary  in  order  to  make  clear  all  the  different  tissues. 
Sections  from  a  squash  stem  may  well  be  studied  for  this  purpose 
because  the  various  cells  of  the  phloem  are  comparatively  large 


8o 


STRUCTURE   OF   BUNDLE 


and  easily  distinguishable.  It  should  be  stated  that  the  bundle 
of  the  squash  is  peculiar  in  that  a  phloem  region  is  developed  on 
both  sides  of  the  xylem.  The  phloem,  like  the  xylem,  is  also 
characterized  by  large  cells,  which  are  here  known  as  sieve  tubes, 
because  these  cells  are  tube-like  structures  with  the  cross  walls 
perforated  like  a  sieve  (Fig.  42,  s).  A  small  cell,  the  accom- 
panying cell,  is  associated  with  the  sieve  tube.  Usually  a  vary- 
ing amount  of  parenchyma  also  occurs  in  the  phloem  and  often 
thick  walled  stereome  fibers  (Fig.  42,  st).  Between  the  xylem 
and  phloem  is  a  region  of  very  delicate  and  regularly  constructed 
cells,  the  cambium  (Fig.  42,  c).  The  growth  of  the  bundle  in 
diameter  and,  in  fact,  of  the  entire  stem  is  brought  about  very 
largely  by  the  formation  of  new  cells  through  the  division  of  the 


FIG.  43.  Longitudinal  section  of  the  bundle  shown  in  Fig.  42:  x,  xylem; 
ph,  phloem;  p,  pith;  v~v'",  annular,  spiral,  scalariform  and  pitted  vessels; 
c,  cambium;  s,  sieve  tubes;  ac,  accompanying  cells;  st,  stereome. — H.  O. 
Hanson. 

cells  in  the  cambium.  The  cross  section  of  the  bundle  reveals 
the  arrangement  and  distribution  of  the  various  tissues  but  it 
will  be  necessary  to  examine  a  section  taken  parallel  with  the 
length  of  the  stem,  i.  e.,  a  longitudinal  section,  in  order  to  arrive 
at  an  understanding  of  the  structure  and  character  of  the  cells 
themselves.  Fig.  43  shows  such  a  section  of  a  bundle.  The 


NATURE   OF   PLANTS  81 

vessels  are  now  seen  to  be  tubular  structures  with  peculiar 
thickenings  of  their  inner  walls  that  assume  the  form  of  rings 
(annular  vessels),  spirals  (spiral  vessels),  or  the  spirals  may 
branch  more  or  less  (reticulate  vessels),  and  often  to  such  an 
extent  that  they  cover  the  entire  surface  of  the  wall  with  the 
exception  of  numerous  small  spaces,  pores,  thus  forming  the 
pitted  vessels  (Fig.  43,  »'").  These  peculiar  sculpturings  prevent 
the  crushing  in  and  closing  of  these  tubes  and  the  annular  and 
spiral  thickenings  also  permit  considerable  elongation  of  such 
ducts  which  would  not  be  possible  if  the  thickening  were  more 
uniform  over  the  wall.  It  will  be  noticed  that  the  later  formed 
ducts  are  characterized  by  reticulate  or  pitted  walls  since  this  type 
of  thickening  must  occur  after  the  elongation  has  taken  place. 
The  ducts  are  composed  of  elongated  cells  but  owing  to  the 
absorption  of  the  majority  of  the  cross  walls  the  ducts  finally 
come  to  resemble  hollow  tubes  that  often  run  for  considerable 
distances  through  the  stem  without  any  cross  walls  at  all.  The 
smaller  cells  of  the  xylem  assume  various  shapes.  Some  of 
them,  the  tracheids,  have  pointed  ends  and  are  characterized  by 
markings  similar  to  the  ducts.  Strengthening  fibers,  similar  to 
those  noted  in  the  cortex,  are  of  common  occurrence  and  also 
short  cells  with  blunt  ends,  wood  parenchyma.  The  ducts  and 
tracheids  soon  lose  their  cell  contents  but  continue  to  function 
in  the  conduction  of  water.  The  wood  parenchyma  and  other 
living  cells  retain  their  vitality  for  several  years  and  generally 
function  as  storage  cells.  It  is  not  improbable  that  they  may 
furnish  some  of  the  energy  required  to  force  the  water  through 
the  ducts  and  tracheids.  It  is  significant  that  living  tissue  is 
always  associated  with  these  water-conducting  cells  of  the  stem. 
Passing  now  to  the  phloem  we  see  that  the  sieve  tubes  are  made 
up  of  elongated  cells  with  perforated  or  sieve-like  cross  walls  (Fig. 
43,  s).  These  minute  openings  adapt  these  cells  to  the  transport 
of  albuminous  substances  which  do  not  readily  diffuse  through 
cell  walls.  The  accompanying  cells  (Fig.  43,  ac)  which  are  cut 
off  from  the  sieve  tubes  probably  assist  them  in  this  work.  At 
any  rate  they  have  abundant  cell  contents  and  contain  nuclei, 
whereas,  singularly  enough,  the  nuclei  of  the  sieve  tubes  soon  dis- 


82  STRUCTURE   OF   BUNDLE 

. 
appear  although  the  rest  of  the  protoplasm  of  the  cells  remains 

active  for  one  or  more  seasons.  Elongated  parenchyma  cells  are 
associated  with  the  sieve  tubes  and  accompanying  cells,  serving 
chiefly  for  the  transport  and  the  temporary  storage  of  the  more 
readily  diffusible  carbohydrates.  Fibrous  cells,  stereome,  are 
also  of  common  occurrence  in  the  phloem  (Fig.  43,  st).  The 
cambium  is  composed  of  very  delicate  cells,  owing  to  the  fact 
that  in  this  region  cell  division  is  taking  place  and  the  cells 
consequently  have  not  yet  attained  their  characteristic  form  (Fig. 
43,  c).  We  may  best  gain  an  idea  of  the  growth  of  the  cambium 
if  we  consider  it  as  a  s:ngle  layer  of  cells  between  the  xylem  and 
phloem.  Each  of  these  cells  divides  into  two  daughter  cells 
(Fig.  44),  u.-ually  the  inner  cell  (i.  e.,  the  cell  next  to  the  xylem) 
develop?  into  a  duct,  tracheid,  or  other  element  of  the  xylem, 
while  the  outer  cell,  after  increasing  to  the  original  size  of  the 
cambium  cells,  divides  into  two  cells  as  at  first.  This  process 
is  repeated  again  and  again,  usually  the  inner  cell  growing  into 
one  of  the  cells  of  the  xylem  while  the  outer  cell  retains  the 


FIG.  44.  Diagram  showing  the  mode  of  division  of  the  cambium  cells. 
The  cambium  cell  is  shaded  to  distinguish  it  from  the  cells  derived  from  it. 
Note  in  the  last  division  at  the  right  that  the  inner  daughter  cell  becomes 
the  cambium  cell  while  the  outer  develops  into  a  phloem  cell. 

power  of  further  division.  Less  commonly  the  reverse  method 
of  growth  takes  place  and  we  have  the  outer  cell  enlarging  and 
forming  one  of  the  cells  of  the  phloem  while  the  inner  cell  acts 
as  a  cambium  cell.  In  this  way  new  cells  are  added  to  the  xylem 
and  phloem  and  the  enlargement  of  the  vascular  bundle  is 
effected. 

35.  The  Conducting  System  of  the  Plant. — These  vascular 
bundles  extend  from  the  root  up  through  the  stem  and  branches 
to  the  leaf  where  they  divide  again  and  again,  reaching  all  parts 
of  it.  In  this  way  the  bundles  become  much  reduced  in  size, 
the  free  end  of  the  vein  often  consisting  of  a  single  tracheid  to 


NATURE  OF   PLANTS  83 

conduct  the  crude  materials  to  the  surrounding  cells  and  elon- 
gated parenchyma  cells  to  collect  the  manufactured  foods  (Fig. 
5,  /).  We  now  see  how  admirably  the  vascular  bundles  are 
adapted  to  the  transport  of  fluids.  Thejcylern  carries  the  sub- 
stances received  from  the  root  hairs  to  all  parts  of  the  plant 
body  quickly  and  with  a  minimum  expenditure  of  energy  because 
oFthe  elongated  character  of  its  cells  and  the  absorption  of  many 
of  the  cross  walls  of  its  cells.  So  the  manufactured  foods  lo- 
cated at  any  point  in  stem  or  leaf  are  gathered  up  and  distributed 
by  the  phloem,  the  more  soluble  and  diffusible  substances  being 
handled  in  part  by  the  thin-walled  parenchyma,  while  non- 
diffusible,  albuminous  substances  can  readily  be  transported  . 
from  cell  to  cell  through  the  perforations  of  the  sieve  tubes. 

36.  The  Strengthening  Tissues  of  the  Stem. — Attention  may 
be  directed  at  this  point  to  the  perfection  of  the  arrangements 
that  give  rigidity  to  the  stem.  The  delicate  cells  of  very  young 
stems  or  of  any  other  part  of  the  plant  are  distended  by  fluids 
which  they  absorb.  In  this  way  a  very  considerable  force,  known 
a.s  the  turgor  of  the  cell,  is  exerted  that  may  amount  to  over  200 
pounds  to  the  square  inch.  This  pressure  at  first  gives  sufficient 
rigidity  to  the  rapidly  elongating  stem,  but  as  it  increases  in 


FIG.  45.     Common  form  of  girder. 

size  and  the  strain  upon  it  becomes  more  considerable,  thick- 
walled  collenchyma  cells  are  formed  that  keep  pace  in  their 
growth  with  the  elongation  of  the  stem.  When  elongation  finally 
ceases,  tough  fibers  of  stereome  appear  which  are  able  to  meet 
the  increasing  strain  upon  the  stem  due  to  the  formation  and 
•development  of  its  various  organs.  These  strengthening  tissues, 
with  which  must  also  be  included  the  tissues  of  the  xylem,  are 
arranged  with  the  same  mechanical  effects  as  are  employed  in 
the  construction  of  buildings,  bridges,  etc.  In  bending  a  beam 


84 


STRENGTHENING   DEVICES 


the  strain  falls  especially  on  the  convex  and  concave  surfaces. 
The  cells  will  be  compressed  on  the  concave  side  and  stretched 
on  the  convex  side,  while  the  tissues  in  the  center  will  be  subject 
to  the  least  disturbance.  For  this  reason  girders  are  strength- 
ened on  the  surfaces  receiving  the  strain  and  weakened  in  the 
center  (Fig.  45).  So  also  given  a  certain  amount  of  material 


FIG.  46.  Cross-section  of  stems  showing  arrangement  of  strengthening 
tissues:  A,  burdock  with  strands  of  collenchyma — st,  at  periphery;  v,  vas- 
cular bundles.  B,  sweet  clover,  showing  a  similar  arrangement.  C,  moon- 
seed  showing  cylinder  of  stereome  fibers  outside  of  vascular  bundles.  D, 
rush  with  strands  of  stereomefibers  at  periphery^of  stem  and  also  on  inner 
and  outer  sides  of  the  bundles;  v,  vascular  bundles;  st,  stereome. 

this  is  more  effectively  distributed  in  the  form  of  a  hollow  column 
than  in  a  solid  column  since  less  strain  falls  upon  the  center  of 
the  column.  Therefore  in  stems  we  find  the  strengthening  tissue 


NATURE  OF  PLANTS  85 

very  economically  distributed  at  the  periphery  either  as  bands 
or  strands  of  collenchyma  and  stereome  fibers  and  these  struc- 
tures are  frequently  reinforced  by  additional  mechanical  tissue 
in  the  phloem  or  just  outside  of  it  (Fig.  46).  The  vascular 
bundles  themselves  frequently  furnish  excellent  examples  of  the 
girder,  masses  of  stereome  being  found  on  the  outside  of  the 
phloem  and  on  the  inside  of  the  xylem  (Fig.  46,  U).  The  resis- 
tance of  these  tissues  to  cutting  and  breaking  affords  abundant 
evidence  of  their  hardness  and  rigidity.  It  may  be  surprising 
to  note  that  the  sustaining  strength  of  the  stereome^  fibers  is 
equal  to  that  of  the  best  wrought  iron  or  hammered  steel  Awhile 
their  ductility  is  ten  to  fifteen  times  that  of  iron.  The  superior 
quality  of  these  tissues  and  the  perfection  of  their  arrangement 
result  in  structures  that  can  not  be  approximated  in  any  of  our 
buildings.  The  height  of  the  tallest  chimneys  scarcely  exceeds 
the  diameter  of  their  bases  more  than  fifteen  times  but  many 
stems  of  rushes  and  grasses  exceed  the  diameter  of  their  bases 
from  200  to  500  times.  It  would  be  impossible  with  any  metal 
to  construct  a  column  of  the  same  length  and  weight  and  having 
the  same  rigidity,  elasticity,  and  resisting  power  as  these 
We  gain  an  idea  of  the  toughness  and  durability  of  these 
when  we  consider  that  linen,  rope,  matting,  etc.,  are  manufac- 
tured from  them.  In  stems  that  increase  greatly  in  diameter, 
as  our  trees,  the  mechanical  tissues  are  confined  to  the  wood  or 
xylera.  These  are  to  be  sure  nearly  solid  columns,  but  even 
here  there  is  sufficient  strengthening  tissue  in  the  xylem  at  the 
periphery  of  the  stem  to  support  the  trunk,  as  is  often  attested 
by  the  sturdy  character  of  trees  that  have  become  hollow  through 
decay. 

37.  The  Secondary  Growth  of  the  Stem.  —  The  arrangement 
of  the  tissues  as  outlined  above,  termed  the  primary  growth, 
remains  practically  unchanged  in  some  annual  plants  but  in  such 
forms  as  live  on  from  year  to  year  or  that  increase  materially 
in  diameter  there  results  extensive  alterations  in  the  structure 
of  the  stems  owing  to  the  formation  of  new  tissues,  especially 
in  the  region  of  the  cortex  and  in  the  vascular  bundles.  The 
epidermis  furnishes  sufficient  protection  to  such  stems  as  do  not 


stems^^[^U€^i 
/fibers) 


86  NATURE   OF   CORK  TISSUE 

increase  materially  in  size,  such  as  the  majority  of  our  annual 
plants;  but  in  long  lived  stems,  as  shrubs  and  trees,  where  growth 
goes  on  from  year  to  year  the  epidermis  is  not  able  to  keep  pace 
with  the  growth.  ,  p  +3rk 

38.  Cork  Tissue. — To  meet  this  condition  a  new  tissue,  the 
cork,  is  developed  from  certain  cells  called  the  cork  cambium, 
usually  situated  near  the  epidermis  (Fig.  47).  The  cells  of  the 
cork  cambium  divide  much  after  the  manner  noted  in  the  cam- 
bium of  the  vascular  bundle,  but  there  is  this  difference,  the  outer 
cell  of  the  two  daughter  cells  becomes  a  cork  cell  while  the  inner 
cell  remains  capable  of  further  division.  Only  rarely  does  the 
outer  daughter  cell  act  as  the  cambium  cell  while  the  inner  cell 
develops  into  one  of  the  cells  of  the  cortex.  As  soon  as  the 
cork  cells  have  reached  their  growth,  a  substance  called  suber 


FIG.  47.     Cross-section  of  the  outer  part  of  the  stem  of  geranium.     The 
cork  cambium,  c,  originating  in  the  cells  immediately  below  the  epidermis. 

begins  to  be  deposited  in  their  walls.  This  completely  changes 
the  properties  of  the  cellulose  walls  and  renders  them  impervious 
to  fluids  and  gases.  So  the  cork  cells  have  the  same  physical 
properties  as  the  cuticle  of  the  epidermis  and  owing  to  the 
continued  activity  of  the  cork  cambium  they  are  able  to  keep 
pace  with  the  growth  of  the  stem.  It  is  evident  that  the  cork 
cells  must  die  as  soon  as  they  become  impervious  to  fluids,  and 
it  must  also  follow  that  all  cells  lying  outside  of  these  cork  cells 
will  die  since  no  fluids  can  reach  them  from  the  vascular  bundles. 
These  dead  cork  cells  give  the  characteristic  aspect  to  the  outer 
bark  of  trees  and  we  would  naturally  come  to  think  of  the  coarse, 
dark  bark  as  composed  of  rather  thick  cells.  As  a  rule,  however, 
the  cork  cells  are  quite  delicate  and  compactly  put  together  (Fig. 
48)  and  the  dark  color  is  more  usually  due  to  a  discoloration  of 
their  walls  or  to  the  dried  remains  of  the  cell  contents.  The 


NATURE  OF   PLANTS  87 

furrows  and  seams  that  occur  in  the  bark  of  most  trees  are  caused 
by  the  continued  activity  of  the  cork  cambium  and  the  cambium 
of  the  vascular  bundles  which  adds  each  season  new  cells  to  the 
stem  and  so  pushes  out  the  cork  cells.  In  this  way  it  comes  about 


FIG.  48.  FIG.  49. 

FIG.  48.  Cross-section  of  cells  of  bottle  cork  showing  the  delicate  character 
of  cork  cells. 

FIG.  49.  Scale  bark  of  pitch  pine.  The  crescent-like  lines  in  the  bark 
show  the  successive  positions  of  the  cork  cambium. 

that  the  cork  cells  are  pushed  further  and  further  from  the  center 
of  the  stem  and  since  they  are  not  capable  of  dividing  they  are 
pulled  apart,  forming  the  characteristic  furrows  in  the  bark. 
It  will  often  be  noticed  that  the  bark  of  some  trees,  as  the  cone- 
bearing  trees,  sycamore,  etc.,  cleaves  off  in  shell-like  scales. 
This  is  due  to  the  formation  of  a  new  cambium  that  joins  on  to 


FIG.  50.  Cross-section  of  the  outer  part  of  a  stem  showing  the  early 
development  of  a  lenticel.  Note  the  irregular  character  and  loose  arrange- 
ment of  the  cells  below  the  stoma  and  the  cork  cambium,  c,  extending  out  on 
either  side  of  the  lenticel. 

the  old  cambium  in  the  form  of  crescents  (Fig.  49).  Conse- 
quently as  this  bark  is  pushed  out  it  breaks  along  these  successive 
crescent  shaped  cambiums  and  finally  cleaves  off  in  scales.  In 


88 


NATURE   OF   LENTICELS 


some  trees  the  cork  cells  break  off  about  as  fast  as  they  are  formed, 
so  that  a  comparatively  thin  layer  of  cork  cells  remains  attached 
to  the  trunk.  Such  trees  have  a  smooth  bark,  as  in  the  beech. 
In  other  cases  the  cork  cells  are  formed  in  great  abundance, 
and  owing  to  the  adhesion  of  the  cells,  thick  layers  of  cork  are 
formed,  as  in  the  oaks,  and  giant  trees  of  the  Pacific. 

It  is  evident  that  this  mantle  of  impervious  cork  cells  would 
tend  to  prevent  the  access  of  the  atmosphere  to  the  stem.  We 
have  seen  that  all  living  cells  respire.  In  many  instances  the  air 
spaces  extending  from  the  leaves  to  all  regions  of  the  stem  are 


FIG.  51.  FIG.  52. 

FIG.  51.  Later  development  of  a  lenticel:  c,  cork  cambium;  i,  intercel- 
lular space. 

FIG.  52.  Surface  view  of  lenticels:  A,  lenticels  on  branch  of  horse-chestnut 
appearing  as  minute  brownish  swellings.  B,  old  lenticels  on  white  birch 
appearing  as  dark  lens-shaped  streaks. 

sufficient  to  bring  about  an  adequate  interchange  of  gases  but  in 
young  stems  where  growth  is  vigorous  and  where  therefore 
respiration  is  considerable,  special  devices  are  required  to  bring 
the  living  cells  of  the  stem  into  more  direct  communication  with 
the  air.  This  work  is  effectively  accomplished  in  herbaceous 
stems  by  the  stomata,  but  in  stems  characterized  by  the  forma- 
tion of  cork  it  is  noticed  that  the  cells  just  below  the  stomata 
begin  to  divide  and  form  a  rather  loose  mass  of  cells  that  lift  up 
and  finally  rupture  the  tissues  about  the  stomata,  thus  forming  a 
small,  lens-shaped  outgrowth  on  the  surface  of  the  young  stem, 
called  a  lenticel  (Figs.  50;  52,  A).  Soon  this  growth  becomes 


NATURE   OF   PLANTS  89 

localized  in  a  layer  of  cells  situated  on  the  inside  of  the  lenticel 
(Fig.  51).  This  layer  of  cells  is  a  part  of  the  cork  cambium, 
but  strangely  enough  only  loose  cells  are  added  to  the  lenticel 
during  each  year's  growth  and  consequently  a  passage  way  is 
kept  open  to  the  living  cells  within  the  stem.  The  lenticels 
appear  as  minute  points  upon  the  surface  of  young  stems  but 
upon  old  trunks  they  often  become  greatly  elongated,  forming 
the  characteristic  bands  on  the  bark  of  the  birch  and  cherry,  etc. 
(Fig.  52,  B).  In  bottle  cork,  derived  from  the  cork  oak  of  the 
Mediterranean,  the  lenticels  appear  as  minute  lines  often  errone- 
ously referred  to  as  worm  holes.  Usually  as  the  cork  layer 
increases  in  thickness  the  cortical  cells  become  less  active,  lose 
their  chlorophyll  and  the  cork  cambium  closes  the  lenticels  by 
forming  compact  cork  cells  instead  of  the  loose  cells  of  the 
lenticels.  This  same  closure  of  the  lenticels  is  frequently  seen 
in  the  fall  but  the  renewal  of  growth  in  the  spring  results  in  the 
formation  of  new  loosely  related  cells  which  rupture  the  over- 
lying cork  layer  and  so  open  the  lenticels.  So  it  seems  probable 
that  the  lenticels  arise  owing  to  the  active  respiration  and 
transpiration  of  the  cortical  cells  and,  when  these  functions  di- 
minish, that  the  lenticels  become  closed  and  finally  fail  to  develop 
further. 

39.  The  Cambium  Cylinder. — Let  us  now  consider  the  changes 
that  are  effected  in  the  vascular  bundles  through  the  activity  of 
the  cambium.  The  increase  in  the  diameter  of  the  stems  of 
many  annual  plants  and  especially  of  our  trees  and  shrubs  is 
largely  brought  about  by  the  formation  of  new  cells  derived 
from  the  cambium.  While  the  vascular  bundles  are  very  small 
it  will  be  noticed  that  the  parenchyma  cells  separating  the  vascu- 
lar bundles  begin  to  divide  so  as  to  form  a  line  of  cells  connecting 
the  cambium  of  each  bundle  (Fig.  53).  In  some  cases  these 
divisions  are  at  first  somewhat  irregular  but  soon  the  growth 
results  in  the  formation  of  cells  with  parallel  walls  and  in  this 
way  a  line  of  regular  cells  is  formed  which  in  cross  section  appear 
as  a  band  or  ring  of  cells  but  in  longitudinal  extent  they  constitute 
a  cylinder.  These  cells  are  therefore  termed  the  cambium  cylin- 
der and  they  continue  to  divide  as  already  noted  in  the  cambium 
7 


90  THE   ANNUAL   GROWTH 

of  the  bundles  (Fig.  54).     Consequently  new  cells  are  now  added 
not  alone  to  the  vascular  bundles  but  also  along  the  entire  extent 


ph 


FIG.  53.  Cross-section  of  a  stem  of  castor  bean  showing  the  formation  of 
the  cambium  between  two  vascular  bundles:  x,  xylem;  ph,  phloem;  c,  cam. 
bium  of  the  bundle.  The  faint  lines,  ic,  are  the  first  divisions  of  the  parenchyma 
cells  between  the  bundles  that  result  in  the  formation  of  the  cambium  cylinder. 

of  the  cambium  cylinder.  This  growth  results  in  the  formation 
of  a  layer  of  xylem  on  the  inside  of  the  cambium  cylinder  and  a 
layer  of  phloem  on  the  outside,  and  so  brings  about  the  principal 


cam 


FIG.  54.  Cross-section  of  a  stem  of  castor  bean  in  which  the  formation  of 
the  cambium  cylinder,  cam,  as  a  ring  of  regular  cells,  has  been  completed; 
p,  pith;  v,  vascular  bundles;  st,  stereome;  c,  cortex.  Compare  Fig.  40. — 
H.  O.  Hanson. 


NATURE   OF   PLANTS  91 

enlargement  of  the  stem  (Fig.  55).  This  growth  is  repeated 
each  spring  in  such  plants  as  continue  to  enlarge  from  year  to 
year,  as  in  the  shrubs  and  trees.  The  majority  of  all  the  cells 
that  are  to  be  formed  in  a  year's  growth  are  cut  off  from  the 
cambium  cylinder  early  in  the  spring,  usually  by  the  last  of  April. 
The  majority  of  these  cells  become  xylem  cells  while  comparatively 
only  a  few  of  the  cells  are  added  to  the  phloem.  Consequently  the 


cam. 


FIG.  55.  Cross-section  of  stem  of  castor  bean  three  weeks  older  than  one 
shown  in  Fig.  54.  Note  the  changes  that  have  occurred  in  the  stem  and 
especially  the  numerous  cells  that  have  been  added  to  the  xylem.  ck,  cork; 
ph,  phloem;  cam,  cambium  cylinder. — H.  O.  Hanson. 

xylem  increases  faster  than  the  phloem  and  forms  the  bulk  of  the 
tissues  of  the  stem.  The  amount  of  the  xylem  added  to  the  stem 
each  year  is  generally  indicated  by  bands  or  annual  rings  (Fig. 
56).  This  is  due  to  the  fact  that  the  first  cells  formed  in  the 
spring  have  thinner  walls  and  often  contain  a  great  many  ducts, 
whereas  the  later  formed  cells  are  for  the  most  part  small  and  pro- 


92  ANNUAL   RINGS 

vided  with  thicker  walls.  So  there  is  a  sharp  contrast  between 
the  small  thick-walled  cells  of  the  summer  wood  and  the  thinner 
walls  and  larger  cells  of  the  spring  wood  (Fig.  57).  This  ap- 
pears to  the  eye  as  a  band,  the  annual  ring  (Fig.  56).  The 
difference  in  the  structure  of  the  cells  of  the  spring  and  summer 
wood  is  doubtless  due  to  the  seasonal  changes  that  are  charac- 
terized by  the  annual  spring  and  summer  growing  periods  and 
by  the  winter  rest.  At  least  tropical  woods  grown  under  uniform 


FIG.  56.  FIG.  57. 

FIG.  56.  Diagram  of  a  cross-section  of  a  stem  of  black  oak  four  years  old ; 
p,  pith;  I,  2,  3,  4,  annual  rings  of  xylem;  c',  cambium  cylinder;  ph,  phloem; 
cr,  cortex;  ck,  cork;  m,  rays. 

FIG.  57.  Magnified  view  of  a  portion  of  one  of  the  bands  of  black  oak  in 
Fig.  56,  showing  the  thick-walled  summer  wood  succeeded  by  the  thinner- 
walled  cells  and  vessels.  This  association  of  cells  causes  the  banded  appear- 
ance of  the  annual  rings  of  growth,  m,  ray;  v,  vessels  in  the  spring  wood. 

conditions  do  not  show  the  annual  rings.  The  age  of  a  tree  can 
generally  be  ascertained  by  counting  the  annual  rings.  However, 
two  rings  may  be  formed  in  one  season  owing  to  the  checking 
of  the  growth  by  fire,  severe  drouth,  depredations  of  insects  and 
the  subsequent  recovery  and  renewal  of  growth.  These  annual 
rings  also  reveal  the  life  history  of  the  tree,  broad  bands  indi- 
cating favorable  seasons  and  narrow  bands  telling  of  fires, 


NATURE   OF   PLANTS 


93 


drought,  transplanting,  and  other  factors  that  limit  the  annual 
growth.  They  also  show  that  the  development  of  the  plant  is 
subject  to  the  same  rhythm  of  growth  as  is  seen  in  the  animal. 


ck 


cam 


FIG.  58.  Diagram  of  a  three-year-old  stem  of  basswood  cut  so  as  to  show 
the  structure  in  cross-section,  C,  in  radial  section,  R,  and  in  tangential  section, 
T;  c,  cortex;  ck,  cork;  ph,  phloem  with  darker  bands  of  stereome;  me,  ray  in 
cross-section;  mr,  ray  in  radial  section;  mt,  ray  in  tangential  section;  /, 
lenticels;  cam,  cambium  cylinder;  p,  pith. — H.  O.  Hanson. 

Namely  the  thickness  of  the  rings  increases  yearly  up  to  a  certain 
age,  after  which  time  there  is  a  gradual  retardation.  So  the 
tree  has  its  youth,  maturity  and  old  age. 


94  MEDULLARY   RAY 


Ray.  —  Another  characteristic  of  most  woody  stems  as 


seen  in  cross  section  is  the  series  of  delicate  lines,  the  rays,  often 
termed  the  medullary  rays,  that  radiate  from  the  center  of  the 
stem  (Fig.  58,  me).  Some  extend  from  near  the  pith  through  the 
phloem  and  others  originate  in  the  various  annual  zones  of  the 
xylem  and  extend  partially  through  the  phloem.  When  mag- 
nified these  rays  are  usually  seen  to  consist  of  rather  thin- 
walled,  oblong  cells  and  varying  in  width  from  one  to  a  few  cells 
(Figs.  57,  m\  58,  me).  In  a  longitudinal  section  cut  parallel 
to  the  rays,  called  a  radial  section,  they  appear  as  bars  of  oblong 
cells  running  at  right  angles  to  the  elongated  cells  of  the  xylem 
(Figs.  58,  mr\  59,  A),  while  in  longitudinal  sections  cut  at  right 


FIG.  59.  Relation  of  the  rays  to  the  xylem  cells  in  pitch  pine:  A,  radial 
section  showing  the  elongated  cells  or  tracheids,  t,  marked  by  circular,  thin 
places  on  bordered  pores,  p;  m,  ray  of  two  cells  accompanied  by  tracheids, 
mt.  B,  tangential  section  showing  rays  one  cell  broad  and  three  to  nine 
high — p,  bordered  pores. 

angles  to  the  rays,  called  a  tangential  section,  we  see  that  the 
cells  of  the  rays  are  arranged  in  a  lens-shaped  group  (Figs.  58,  mt] 
59,  B).  The  rays  are  especially  of  great  importance  in  the  dis- 
tribution and  often  also  in  the  storage  of  foods.  Their  structure 
and  arrangement  in  the  stem  are  admirably  adapted  for  this  work. 
Their  thin  walls  can  readily  withdraw  from  the  ascending  current 
in  the  xylem  as  much  water  as  is  needed  for  the  phloem  and  cor- 


NATURE  OF   PLANTS  95 

tex.  Tracheids  are  often  associated  with  the  ray  cells  to 
further  increase  their  absorbing  power  (Fig.  59,  A,  mt).  Par- 
ticularly is  this  true  in  the  spring  wood  where  the  ray  cells  also 
often  become  greatly  enlarged  in  order  to  absorb  the  large  volume 
of  water  required  at  this  time  of  year.  In  the  same  way  the 
carbohydrates  and  albuminous  substances  transported  through 
the  phloem  are  withdrawn  through  the  ray  cells  and  conveyed 
to  the  active  cambium  and  the  growing  cells  of  xylem. 

The  value  of  oak  and  other  woods  for  interior  decoration  is 
materially  increased  by  the  wood  rays.  The  trees  are  sawed 
into  timber  in  such  a  way  as  to  expose  the  wood  rays  to  the 
best  advantage,  a  process  called  quartering  (Fig.  60).  Conse- 


FIG.  60.  Diagram  showing  a  common  method  of  sawing  in  producing 
quartered  oak.  The  log  is  first  squared  to  remove  the  sap  wood  and  then  a 
number  of  boards,  depending  upon  the  size  and  grain  of  the  tree,  are  removed 
at  a  and  b.  These  are  the  best  boards  obtained  from  the  log,  being  broad  and 
approximately  radial  and  therefore  showing  the  wood  rays  to  good  ad- 
vantage. The  remaining  portions  of  the  log  are  cut  radially,  as  shown  at  c. 
One  of  the  ends  of  quartered  lumber  is  oblique  and  must  be  squared,  as  shown 
in  the  diagram. 

quently  the  rays  appear  as  flecks  and  bands  upon  the  surface  of 
the  board  and  vary  greatly  in  character  owing  to  the  angle  of 
cutting  and  to  the  irregularities  of  the  trunk.  The  wood 
rays  can  always  be  detected  in  the  grain  of  wood  because  they 
run  across  or  at  right  angles  to  the  xylem  cells  (Fig.  58,  mr). 
As  a  rule  the  wood  cells  of  the  stem  remain  alive  for  only  a  few 
years.  This  fact  is  usually  apparent  in  any  log  or  stump  where  it 
will  be  seen  that  the  outer  annular  rings  are  of  a  lighter  color,  the 
sap  wood,  and  the  older  parts  are  of  a  darker  color,  the  heart 
wood.  The  sap  wood  is  active  in  the  transport  of  water  and 
contains  many  living  cells.  For  this  reason  it  is  not  so  valuable 


96  OTHER  TYPES   OF  STEMS 

for  lumber.  The  heart  wood  is  composed  of  dead  cells  and 
while  the  most  serviceable  for  building  purposes,  it  is  of  little 
use  to  the  tree  as  we  often  see  a  vigorous  tree  whose  heart  wood 
has  been  largely  destroyed  by  decay.  We  ought  not  to  leave  this 
vascular  system  of  the  plant  without  again  emphasizing  the  per- 
fection of  it.  First  there  is  the  completeness  of  the  system,  ex- 
tending from  the  root  to  all  parts  of  the  leaf.  As  soon  as  the  root 
hairs  begin  to  form  and  function  there  is  already  at  that  point  the 
beginnings  of  one  of  the  terminals  of  the  system  while  the  other 
terminal  is  also  appearing  in  the  leaves  while  they  are  forming  and 
still  concealed  in  the  buds.  Then  the  elongated  cells,  those  of 
the  xylem  being  in  addition  largely  without  living  cell  contents, 
ensures  a  rapid  transport  of  material.  The  weak  feature  of  this 
system  is  the  terminals  for  here  the  materials  must  not  only  pass 
through  the  numerous  walls  of  the  short  cells  but  also  through 
the  still  more  impermeable  membranes  of  the  living  matter  of 
these  cells.  But  once  the  material  is  landed  in  the  elongated 
cells,  then  follows  a  rapid  transport.  Therefore  it  is  the  develop- 
ment of  this  system  that  makes  possible  the  stature  of  plants. 
Without  it,  plants  must  remain  small  and  in  close  reach  of  their 
crude  supply. 

41.  Other  Types  of  Stems. — The  cone  bearing  trees,  such  as 
the  pines  and  spruces,  have  essentially  the  same  arrangement 
of  tissues  and  mode  of  growth  as  noted  above  in  the  dicotyledons. 
The  xylem,  however,  consists  entirely  of  tracheids  (Fig.  59) 
with  the  exception  of  a  few  small  spiral  vessels  that  are  formed 
as  the  first  cells  of  the  vascular  bundles. 

The  monocotyledons,  plants  distinguished  usually  by  their 
parallel  veined  leaves  and  single  seed  leaf,  like  the  palms,  lilies, 
grasses,  etc.,  are  characterized  by  stems  that  do  not  increase 
materially  in  diameter.  Growth  is  largely  confined  to  the  top 
of  the  stem  and  consequently  it  can  only  elongate,  forming  a 
very  regular,  columnar  trunk.  The  reason  of  this  is  apparent 
when  cross  sections  of  such  stems  are  examined  (Fig.  46,  D). 
The  vascular  bundles  are  more  or  less  scattered  throughout  the 
stem.  The  cause  of  this  arrangement  is  seen  in  the  seedling 
stage  of  the  plant.  Here  we  see  the  vascular  bundles  arising 


NATURE  OF  PLANTS 


97 


through  the  opening  up  of  the  vascular  cylinder  just  as  in  the 
case  of  the  dicotyledones,  page  76.  The  vascular  bundles  thus 
formed,  however,  do  not  long  pursue  a  uniform  course  through 
the  stem,  some  soon  turning  into  the  pith  and  others  bending 
into  the  cortex.  It  is  also  especially  noteworthy  that  there 
usually  no  cambium  separating  the  xylem  and  phloem.  It  will 
be  seen  by  examining  Fig.  61,  that  the  tissues  of  the  bundles  are 


FIG.  61.  Cross-section  of  a  single  vascular  bundle  of  corn  stem:  ph, 
phloem;  x,  small  cells  of  the  xylem;  vt  vessels  of  xylem;  st,  stereome  that 
forms  a  sheath  about  the  bundle ;  p,  parenchyma  of  the  stem. 

of  a  similar  character  and  have  the  same  arrangement  as  in  the 
dicotyledons  (Fig.  42)  save  for  the  absence  of  the  cambium  which 
prevents  the  addition  of  new  cells  to  the  xylem  and  phloem. 
Consequently  there  can  be  no  considerable  increase  in  the  dia- 
meter of  the  stem  and  no  necessity  therefore  for  the  protective 
layer  of  cork  cells. 


98 


ELONGATION   OF   THE   STEM 


42.  Apical  Growth  of  the  Stem.— All  stems  are  characterized 
by  a  more  or  less  extended  apical  growth.  The  changes  pro- 
duced in  stems  by  the  growth  of  the  cambium  only  result  in 
increasing  their  diameters  but  the  elongation  of  all  stems  is 
effected  by  the  formation  of  new  cells  at  the  apex  of  the  stem. 
Fig.  62  shows  the  general  features  to  be  noticed  in  the  tip  of 
a  stem  as  seen  in  longitudinal  section.  New  cells  are  being 
formed  at  the  apex  by  division  just  as  in  the  case  of  the  root. 
Below  this  formative  region  is  the  zone  of  elongation  in  which 
the  cells  gradually  change  in  character  so  that  the  cortical  and 
central  regions  are  apparent  and  in  certain  elongated  cells  we 
note  the  first  indication  of  the  vascular  bundles  (Fig.  62,  x). 


e     c 

FIG.  62.  Longitudinal  section  of  the  tip  of  a  growing  stem:  e,  epidermis 
extending  over  surface  of  the  entire  tip;  a,  formative  region;  b,  upper  portion 
of  the  zone  of  elongation;  c,  cortex;  x,  cells  of  the  central  region  that  by 
further  growth  form  the  vascular  bundles;  /,  first  appearance  of  the  leaves. 

The  leaves  also  originate  in  this  region  through  the  active  divi- 
sion of  the  superficial  cells  of  the  stem.  The  branches  of  the 
stem  develop  somewhat  later  in  the  axils  of  the  leaves,  and  more 
deeply  located  cells  in  the  cortex  co-operate  in  their  formation. 
This  relationship  of  the  leaves  and  branches  to  the  stem  is  shown 
in  Fig,  63,  which  is  a  diagram  of  an  elongating  stem,  showing  the 
relation  of  the  apical  region  of  the  stem  (a  in  Fig.  62),  to  the 
lower  and  older  portion.  The  cells  in  these  young  leaves  and 
branches  by  rapid  division  and  growth  soon  form  the  character- 


NATURE  OF   PLANTS 


99 


istic  tissues  already  noted  in  the  leaves  and  stems;  while  a  cor- 
responding growth  in  the  cortex  and  adjacent  regions  results  in 
the  formation  of  vascular  bundles  that  connect  the  vascular 
bundles  of  the  stem  with  those  of  the  leaves  and  branches.  When 
a  leaf  falls  off,  the  ends  of  these  vascular  bundles  can  be  easily 
seen  in  the  leaf  scar  (Fig.  28,  A)  but  owing  to  their  minuteness 


. 


FIG.  63.  Diagram  of  the  tip  of  a  stem  as  seen  in  longitudinal  section : 
a,  formative  region  corresponding  to  the  part  shown  in  Fig.  61;  /,  b,  leaves 
and  branches  in  various  stages  of  development;  v,  vascular  bundles;  c,  cortex; 
pt  pith. 

it  is  not  an  easy  matter  to  trace  them  through  the  stem  to  the 
point  where  they  join  on  to  the  bundles  of  the  stem.  In  the 
branch,  however,  owing  to  its  size  and  woody  character,  the 
union  with  the  stem  is  very  manifest.  Fig.  64  shows  several 
very  small  branches  that  continued  to  keep  pace  for  one  or  more 


100 


RELATION   OF   BRANCH   TO  STEM 


years  with  the  growth  of  the  stem,  but  eventually  they  were 
killed  by  the  overhanging  branches  and  in  time  became  over- 
grown with  the  annual  layers  of  xylem.  If  the  branch  is  favor- 
ably situated  so  that  it  continues  to  live  then  a  cone-like  struc- 
ture is  developed  (Fig.  64,  a)  since  the  branch  increases  in  girth 
each  year  in  the  same  manner  as  the  stem.  These  branches  run- 
ning through  the  xylem  are  the  cause  of  knots  that  appear  in 
lumber.  If  the  branch  is  living  the  tissues  of  stem  and  branch 


FIG.  64.  Section  through  the  trunk  of  basswood  showing  relation  of 
branches  to  main  stem.  In  the  upper  portion  of  the  figure  are  three  branches 
that  were  killed  after  a  few  years'  growth  by  shading  and  that  have  been 
overgrown  by  the  annual  rings  of  the  wood.  The  branch  shown  at  a  has 
remained  alive  and  increased  in  size  after  the  manner  of  the  main  stem. 

are  closely  bound  together  and  we  have  a  solid  knot,  but  it  not 
infrequently  happens  that  branches  after  living  several  years  die 
and  begin  to  decay  before  they  are  overgrown.  Such  a  limb  will 
produce  in  the  lumber  a  knot  that  is  often  black  and  shaky  or  it 
may  fall  out,  forming  a  knot  hole.  The  lower  branches  of  trees 
growing  in  a  forest  are  quickly  pruned  off  by  the  shade  of  the 
upper  branches.  This  causes  straight,  clean  trunks  to  develop 
and  the  lumber  is  free  of  knots. 

It  follows  from  what  has  been  said  above  that  in  pruning 
trees  the  limbs  should  be  cut  off  close  to  the  trunk.  Painting 
the  cut  surface  does  not  promote  the  healing  of  the  wound 
though  it  may  exclude  organisms  causing  decay.  When  a  wound 


NATURE  OF^  PLANTS^'    :Ji  h;  ;  -  101 


is  made  that  extends  across  the  cambium  as  in  the  cutting  off  of  a 
limb  or  by  a  cut  into  the  trunk  that  removes  the  tissues  as  far  as 
the'  xylem  it  will  be  noticed  that  the  exposed  cambium  is  stimula- 
ted to  form  a  mass  of  delicate  cells,  the  callus,  that  gradually  ex- 
tend over  the  wound  if  this  is  not  too  large.  While  the  callus  is  still 
forming  its  outer  cells  become  changed  to  cork  cells  which  join 
onto  the  old  cork  while  a  new  cambium  layer  is  also  developed  on 
the  inner  side  of  the  callus,  which  unites  with  the  old  cambium, 
thus  continuing  the  cambium  of  the  stem  over  the  surface  of  the 
wound  (Fig.  65,  A).  In  this  way  wounds  are  covered  or  healed 


x    cm 


c  c 

A.  B.  c 

FIG.  65.  Section  of  a  portion  of  a  stem  showing  the  healing  of  a  wound 
caused  by  the  cutting  off  of  a  lateral  branch:  A,  formation  of  the  callus,  cl, 
owing  to  the  renewed  activity  of  the  living  cells  exposed  by  the  wound;  c, 
cortex;  cm,  cambium;  x,  region  of  the  xylem.  B,  a  similar  stem  three  years 
later — c,  cortex;  cm,  cambium;  x',  xylem  added  since  the  wound  was  made; 
o,  position  of  the  cambium  at  the  time  the  branch  was  cut  off;  x,  original 
xylem  of  the  stem.  C,  trunk  from  which  three  branches  have  been  removed, 
showing  the  gradual  covering  of  the  wounds  by  new  tissue. 

and  the  tissue  exposed  by  the  cut  is  gradually  buried  deeper  by 
the  annual  addition  of  new  tissue  derived  from  the  cambium  (Fig. 
65,  B,  C).  Grafting  is  made  possible  owing  to  the  formation  of 
callus  that  unites  the  tissues  of  the  scion  and  stock  (Fig.  66). 
While  grafting  is  performed  in  a  variety  of  ways  the  process 
consists  essentially  in  all  cases  of  bringing  the  cambium  and 
cortex  of  the  cutting  or  scion  in  contact  with  the  corresponding 


IO2 


PRUNING   AND   GRAFTING 


region  of  the  stock.  The  junction  of  the  scion  with  the  stock  is 
sealed  with  grafting  wax  to  prevent  decay  and  the  drying  out 
of  the  exposed  cells  while  the  callus  is  joining  together  the  corre- 
sponding tissues  in  the  two  parts.  When  the  wound  is  healed 
only  a  slight  ring  or  swelling  indicates  the  point  where  the  scion 
was  inserted — so  complete  is  the  union  of  the  vascular  and  cor- 
tical regions.  It  is  evident  that  pruning  and  grafting  should  be 
done  at  a  time  when  the  cambium  is  most  active,  i.  e.,  in  the 
spring.  In  the  case  of  fruit  trees  it  is  now  the  practice  to  defer 
the  pruning  until  some  time  in  July  or  when  the  apical  bud 


FIG.  66.  A  common  method  of  grafting:  A,  insertion  of  two  scions  into 
cleft  of  stock  at  cambium  region.  B,  wound  protected  with  wax  to  prevent 
drying  out  of  tissues. — After  L.  H.  Bailey. 

is  formed,  because  the  cambium  is  still  active  at  this  season  and 
especially  since  the  buds  are  forming.  By  removing  that  portion 
of  the  branch  which  would  only  produce  leaf  buds  a  better 
development  of  the  flower  buds  is  ensured. 

Although  the  organic  union  between  scion  and  stock  is  com- 
plete, each  part  continues  a  separate  and  distinct  existence.  For 
example,  a  scion  from  a  plum  grafted  upon  a  branch  of  a  peach 
tree  will  develop  a  shoot  bearing  plums  and  the  peach  tree  will 
also  continue  its  normal  growth.  Ordinarily  only  closely  related 
forms  may  be  grafted,  as  tomatoes  and  potatoes,  apples  and 
quinces,  etc.  It  must  be  inferred  from  the  above  statement 
that  there  is  no  interaction  between  stock  and  scion.  Usually 
this  is  not  sufficient  to  change  materially  the  character  of  either 
but  there  are  numerous  instances  showing  that  the  chemical  com- 
position of  the  cells  or  even  the  form,  coloration,  and  productive- 


NATURE  OF   PLANTS  103 

ness  of  scion  may  be  modified  by  the  stock.  Fruit  growers  take 
advantage  of  this  in  grafting  the  cuttings  from  seedlings  that 
would  not  bear  for  several  years  upon  fruiting  trees,  with  the 
result  that  the  scion  comes  into  fruitfulness  the  next  season. 
In  the  same  way  it  is  claimed  that  seedlings  may  be  made  to 
bear  earlier  than  they  normally  would  by  grafting  them  with  buds 
from  fruiting  trees. 

43.  Comparison  of  Stem  and  Root. — It  will  be  noticed  that  the 
structure  of  the  growing  tip  of  the  stem  presents  several  striking 
departures  from  that  of  the  root.  In  the  first  place  a  protective 
structure  such  as  the  root  cap  is  not  required  (Fig.  62).  The 
leaves,  and  consequently  the  branches  are  formed  with  great 
regularity  from  the  superficial  tissues  of  the  stem,  whereas  the 
rootlets  are  deep  seated  in  origin  and  irregularly  distributed. 
Furthermore,  the  elongation  of  the  stem  is  distributed  over 
several  centimeters,  while  in  the  root  it  is  confined  to  a  few  milli- 
meters. The  leaves  and  branches  are  distributed  over  the  entire 
zone  of  elongation,  while  lateral  roots  do  not  appear  until  elonga- 
tion has  ceased.  This  prolonged  growth  of  the  tip  of  stems  is  of 
great  advantage  in  bringing  about  the  proper  adjustment  of 
leaves  and  branches.  These  organs  come  into  sharp  competition 
with  each  other  as  well  as  with  other  plants,  and  so  long  as  growth 
continues  there  is  a  possibility  for  them  to  become  adjusted  to  the 
light  without  interfering  with  one  another.  The  mode  of  elonga- 
tion of  stems  and  roots  is  essentially  alike.  Each  has  a  formative 
region  at  the  apex  characterized  by  active  cell  formation  and 
slight  elongation,  back  of  this  is  the  zone  of  rapid  elongation  and 
following  this  is  the  zone  in  which  elongation  is  gradually  ceas- 
ing and  the  tissues  are  assuming  their  characteristic  forms  and 
functions.  It  is  also  important  to  note  that  the  growth  of  the 
cells  on  one  side  of  the  stem  or  root  is  faster  at  a  certain  time 
than  that  of  the  other  cells.  This  will  cause  the  stem  to  be  bent 
towards  the  opposite  side.  This  condition  of  rapid  elongation 
does  not  continue  on  one  side  for  any  considerable  time  but 
slowly  moves  from  point  to  point  around  the  stem  so  that  the 
stem  is  successively  bent  from  side  to  side,  thus  causing  the  apex 
to  travel  in  an  irregular  circle.  By  these  constant  movements, 


104  SENSITIVENESS   OF  STEMS 

nutation,  the  stems  and  roots  are  more  fully  exposed  to  the  vari- 
ous forces  that  influence  their  development  and  consequently 
they  adjust  themselves  to  the  best  advantage  (see  page  33). 

As  a  rule  the  irritability  or  sensitiveness  of  the  stem  is  not  so 
localized  as  in  the  case  of  the  root.  It  is  evident  that  it  is  an 
advantage  to  have  the  entire  growing  surface  of  the  stem  sensi- 
tive to  all  those  stimuli  that  influence  its  growth,  because  by  this 
means  it  will  be  so  influenced  as  to  bring  its  leaves  and  branches 
into  proper  relations  with  the  surroundings.  In  some  plants  to 
be  sure  the  sensitiveness  to  stimuli  is  quite  as  localized  as  in  the 
root;  as,  for  example,  the  tip  of  the  cotyledons  of  many  grasses 
which  have  an  appreciation  of  light  almost  the  equal  of  our  eyes. 
These  cotyledons  will  turn  toward  a  light  too  feeble  to  enable  us 
to  read  the  time  of  day  on  a  watch.  The  significant  fact  to  be 
noted  here  is  that  this  keenness  and  localization  directs  the  plant 
into  a  suitable  place  as  soon  as  it  emerges  from  the  soil.  Grass 
plants  grow  in  thick  colonies  and  therefore  the  stems  come  into 
sharp  competition.  Consequently  it  is  a  necessity  that  the  young 
shoot  on  emerging  from  the  soil  should  be  able  to  appreciate  the 
feeblest  light  and  so  be  able  to  direct  its  growth  through  the 
smallest  openings. 

The  reactions  of  the  stem  to  various  stimuli,  such  as  light, 
gravity,  etc.,  are  quite  as  purposive  as  those  noted  in  the  root. 
Lateral  light,  as  in  the  case  of  plants  growing  in  windows,  acts 
as  a  stimulus  and  causes  a  more  considerable  growth  of  the  cells 
on  the  darker  sides  of  the  stem.  This  results  in  bending  the 
stems  towards  the  light  and  consequently  in  the  exposure  of  the 
broad  blades  of  the  leaves  to  the  light.  So,  also,  when  erect  stems 
are  placed  in  a  horizontal  position  the  stimulus  of  gravity  awakens 
a  more  active  growth  on  the  underside  of  the  stem,  thus  bending 
the  tip  into  an  erect  position  for  securing  light.  Not  all  stems 
and,  indeed,  not  all  parts  of  the  same  plant  body  are  similarly 
influenced  by  these  forces.  Many  plants  tend  to  arrange  their 
stems  at  right  angles  to  the  action  of  gravity  and  light,  as  in  the 
case  of  creeping  and  underground  stems.  So  also  the  lateral 
branches  are  inclined  at  various  angles  to  the  stem.  These  ad- 
justments are  not  to  be  looked  upon  solely  as  the  direct  response 


NATURE  OF  PLANTS  105 

to  one  or  more  stimuli  but  as  the  resultant  of  numerous  interac- 
tions aroused  in  the  plant  by  various  stimuli.  The  entire  plant 
responds  as  a  unit  to  these  forces.  Stimulation  and  the  conse- 
quent growth  in  one  part  has  its  effects  upon  all  other  parts.  As 
a  result  of  this  interaction  there  comes  about  a  correlation  in  the 
growth  of  all  the  parts  that  is  expressed  in  the  symmetry  of  the 
plant  body  and  in  the  perfect  adjustment  of  roots,  stem  and 
branches,  and  leaves  to  its  needs.  Any  window  garden,  forest, 
or  field  furnishes  abundant  evidence  of  the  complicated  and 
varied  nature  of  these  reactions.  An  illustration  of  this  is 
afforded  in  the  removal  of  the  terminal  axis  of  a  tree.  An  ad- 
joining lateral  branch  that  heretofore  has  been  stimulated  to  a 
more  or  less  horizontal  growth,  now  is  so  directed  in  its  develop- 
ment that  it  ultimately  becomes  the  terminal  axis  of  the  tree. 
So  some  parts  are  constantly  being  overshadowed  and  choked 
out  while  others  are  directed  into  favorable  positions  and  perform 
the  work  of  the  destroyed  parts.  In  this  way  there  is  brought 
about  that  weaving  and  twisting  of  stems  and  branches  and  the 
adjustment  of  leaves  that  so  often  suggest  a  conscious  effort  to 
reach  favorable  positions. 

44.  Modifications  of  the  Stem. — In  the  above  discussion  at- 
tention has  been  called  to  the  more  common  characteristics  of 
the  stem.  It  is  well  to  remember,  however,  that  variation  is 
the  law  in  the  plant  world.  No  two  plants  are  alike.  The 
environment  of  the  plant  is  constantly  stimulating  it  and  caus- 
ing it  to  vary.  Frequently  the  variations  are  so  minute  as  to 
escape  our  attention  or  totally  distinct  forms  may  arise.  These 
departures  from  the  parent  type  may  be  of  no  advantage  to  the 
plant  and  it  may  therefore  perish.  On  the  other  hand  the  varia- 
tions may  be  of  such  a  character  as  to  enable  the  plant  to  more 
successfully  compete  with  other  plants  and  as  a  consequence  the 
plant  with  its  helpful  variations  will  survive.  So  it  has  come 
about  that  these  changes  going  on  during  the  past  ages  have 
resulted  in  many  remarkable  modifications.  A  few  of  the  modi- 
fications of  the  stem  that  are  of  especial  advantage  to  the  plant 
and  therefore  spoken  of  as  adaptive  variations  will  be  considered 
in  the  following  paragraphs. 


io6  NATURE   OF  CLIMBING  STEMS 

45.  The  Climbing  Type  of  Stems. — Climbing  stems  are  char- 
acterized by  being  rather  small  and  having  greatly  elongated 
internodes.  Stems  grown  in  the  dark  show  a  similar  develop- 
ment. Perhaps  this  variation  that  we  call  the  climbing  type 
has  been  brought  about  by  competition  with  larger  plants.  The 
feeble  light  has  stimulated  these  stems  so  that  they  attain  a  very 
extended  growth  and  finally  are  able  to  reach  the  light  and 
display  their  leaves.  Many  variations  in  the  structure  and  sensi- 
tiveness of  the  stem  are  associated  with  this  elongation,  all  of 
which  are  designed  in  one  way  or  another  to  enable  the  stem 
to  reach  the  light.  One  type  of  these  varia- 
tions is  seen  in  twining  stems,  as  the  morn- 
ing glory,  bean,  hop,  etc.  Young  twining 
plants  behave  quite  like  the  ordinary  plant. 
The  stems  are  erect  and  actively  nutating, 
the  apex  traveling  through  a  rather  large 
circle  in  one  to  three  hours ;  When  a  cer- 
tain height  has  been  reached  the  stems  are 
stimulated  by  gravity  so  that  their  upper 
portions  grow  more  or  less  horizontally 
(Fig.  67).  This  position  is  a  decided  ad- 
vantage since  the  stem  is  now  revolved 
through  a  larger  circle  and  has  a  greater 
chance  of  coming  into  contact  with  an  ob- 
ject about  which  it  can  twine.  As  soon  as 
the  stem  comes  into  contact  with  any  sup- 

5!°V6M*    Twinx!nS  port  its  nutation  will  cause  it  to  wind  or 
habit  of  wild  bean.  Note 

the  horizontal  position  of  twine  about  it.  The  contact  also  acts  as 
the  upper  portion  of  the  a  stimulus,  causing  the  stem  to  bend  more 

stem  which  results  in  the  energetically .    Certainly  in  many  plants  the 

apex  nutating  through  a     . 

wider  circle.  slze  anc*  roughness  of  the  support  as  well 

as  other  features,  are  important  factors  in 

inducing  the  twining.  At  first  the  coils  are  merely  horizontal 
but  owing  to  the  elongation  of  the  stem  these  coils  are  gradually 
pushed  upwards  and  become  steep  and  very  firmly  bound  around 
the  support.  Twisting  of  the  stems  and  reflexed  bristles  often 
assist  in  anchoring  the  plant  to  its  support.  It  is  interesting  to 


NATURE   OF   PLANTS  107 

note  that  in  the  majority  of  cases  the  stem  twines  about  the  sup- 
port in  left  hand  spirals  or  clockwise;  less  frequently  right  hand 
spirals  are  formed,  as  in  the  hop,  some  honeysuckles  and  knot 
weeds.  Why  the  growth  of  the  stem  results  in  left  or  right  hand 
twining  is  not  known  but  the  fact  remains  that  the  plants  can  not 
be  induced  to  change  their  method  of  twining. 


46.  The  Tendril  Type  of  Stem. — One  of  the  most  interesting 
variations  of  climbing  plants  is  seen  in  those  stems  that  climb 
by  means  of  tendrils.  In  this  type  almost  any  part  of  the  plant 
may  become  modified  so  as  to  act  as  a  tendril  and  bind  the  plant 
to  a  support;  e.  g.,  the  petioles  of  nasturtium  and  clematis, 
stipule-like  outgrowths  in  smilax,  midrib  of  leaf  in  many  members 
of  the  bean  family,  or  modified  branches  as  in  the  grape  vine,  etc. 
The  ordinary  tendrils  as  we  see  them  in  the  gourd,  passion  flower, 
etc.,  are  highly  specialized  stems  that  are  very  sensitive  to  touch. 
These  organs  are  rapidly  nutating,  frequently  completing  two 
or  more  circles  in  an  hour,  and  as  they  approach  maturity  they 
often  become  slightly  curved  and  hooked  (Fig.  68,  A).  Their 
sensitiveness  to  contact,  which  is  often  localized  on  the  concave 
upper  third  of  the  tendril,  also  becomes  so  keen  in  some  forms 
that  a  thread  weighing  one-fiftieth  of  a  gram  will  cause  a  curva- 
ture of  the  tendril  when  placed  upon  it.  In  fact  a  much  smaller 
weight  if  caused  to  vibrate  will  act  as  a  stimulus,  so  that  we 
have  here  an  illustration  of  sensitiveness  to  weight  that  is  keener 
than  our  own.  When  the  sensitive  part  of  a  tendril  is  stroked 
by  a  pencil  or  comes  in  contact  with  any  hard  object,  this  acts 
as  a  stimulus  and  causes  a  very  rapid  growth  upon  the  convex 
side  of  the  tendril.  As  a  result  of  this  growth  the  tip  of  the 
tendril  curves,  forming  a  complete  loop  in  from  a  few  seconds 
to  a  few  minutes.  The  contact  of  the  tip  of  a  tendril  with  any 
object  acts  as  a  continued  stimulus  and  the  tendril  soon  becomes 
tightly  wound  around  it.  As  soon  as  the  tip  is  firmly  bound  to 
the  support  a  new  impulse  is  frequently  transmitted  to  the  free 
portions  of  the  tendril  which  in  a  day  or  so  begin  to  assume  the 
form  of  coils  that  are  often  reversed  in  the  middle  (Fig.  68,  B). 
By  this  unique  device  tendril  bearing  stems  are  firmly  fastened  to 
their  supports  but  at  the  same  time  the  coiling  of  the  tendrils 


io8 


NATURE  OF  TENDRILS 


acts  like  a  spring  permitting  considerable  disturbance  of  the 
vine  without  danger  of  breaking.  This  same  device  is  copied 
in  making  the  attachment  of  the  wires  to  the  telephones  and 
other  pieces  of  apparatus.  From  many  standpoints  the  tendril 
bearing  stem  is  an  admirable  one.  It  is  evident  that  such  stems 
can  reach  the  light  more  directly  and  economically  than  twiners. 
This  doubtless  accounts  for  their  greater  abundance  and  common 


FIG.  68.  FIG.  69. 

FIG.  68.  Tendrils  of  the  bur  cucumber:  A,  hooked  tendrils  in  receptive 
state.  B,  apical  portion  of  tendril  coiled  about  a  branch  and  the  remaining 
portion  of  the  tendril  forming  reversed  coils,  thus  drawing  the  vine  to  the 
supporting  branch. — H.  O.  Hanson. 

FIG.  69.  Branch  of  Japanese  ivy  attached  to  wall  by  means  of  tendrils 
with  adhesive  discs. — J.  O.  Hanson. 

occurrence.  It  is  also  important  to  note  that  the  tendrils  are 
especially  abundant  at  the  end  of  the  branches  where  they  reach 
out  considerably  beyond  the  young  leaves.  Owing  to  the  sway- 
ing of  the  long  free  shoot  of  the  climber  in  the  wind  and  the 
nutations  of  the  tendril  and  shoot  these  sensitive  organs  are 


NATURE  OF  PLANTS  109 

brought  into  contact  with  branch  after  branch  and  thus  the  vine 
is  led  up  to  the  crown  of  the  tree  or  to  the  top  of  the  vegeta- 
tion. The  Virginia  creeper,  Boston  or  Japanese  ivy,  etc.,  have 
peculiar  tendrils  that  grow  away  from  the  light  and  form  ad- 
hesive discs,  instead  of  coils,  that  bind  the  vine  to  the  support 
(Fig.  69).  These  discs  are  caused  by  the  growth  of  the  cells  at 
the  tip  of  the  tendril.  As  soon  as  the  tendril  hits  the  support 
upon  which  the  plant  is  growing,  the  cells  at  the  tip  are  stimulated 
to  grow  out  and  fit  into  every  little  irregularity  and  groove  in 
the  substratum.  Indeed  in  some  vines  these  discs  become  in  this 
way  so  thoroughly  cemented  to  the  crevices  and  irregularities  of 
the  support  as  to  resemble  bits  of  melted  wax.  It  is  very  in- 
teresting to  watch  the  results  of  the  stimulation  induced  by  one 
of  these  tendrils  striking  a  wall  or  other  support.  The  formation 
and  cementing  of  the  disc  are  accomplished  in  about  four  days 
in  the  case  of  the  Virginia  creeper.  Some  plants  like  the  poison 
and  English  ivy  and  many  tropical  plants  climb  by  means  of 
roots.  These  organs,  like  the  disc  forming  tendril,  avoid  the 
light  and  seek  the  dark  nooks  and  corners  in  the  bark  of  trees  or 
rocks  over  which  the  plants  grow. 

47.  Prostrate  and  Creeping  Stems. — It  is  interesting  to  note 
that  a  large  number  of  plants  having  rather  small  and  weak 
stems  like  the  climbers,  have  never  acquired  the  habit  of  climb- 
ing. The  young  shoots  of  these  plants  are  often  erect  at  first 
but  soon  the  weak  stems  bend  over  and  become  prostrate.  While 
these  procumbent  and  creeping  stems  are  at  a  disadvantage  in 
that  they  are  obliged  to  arrange  their  leaves  in  one  plane,  on 
the  other  hand  they  are  not  obliged  to  build  up  the  elaborate 
structures  that  are  necessary  for  the  support  of  the  erect  stems. 
Consequently  plants  of  this  class  are  of  common  occurrence  in 
old  fields  and  unfertile  soils  since  they  can  subsist  on  a  meager 
supply  of  food.  They  are  also  protected  against  injury  and 
therefore  adapted  to  wind  swept  sandy  plains,  to  rocky  hills  and 
mountains,  and  to  districts  visited  by  heavy  snows.  The  ma- 
jority of  these  plants  also  have  a  very  decided  advantage  owing 
to  the  fact  that  certain  nodes  form  roots  and  buds  that  develop 
into  new  plants.  In  some  cases,  stolons,  only  the  terminal  bud 


I io  RHIZOME  TYPE   OF  STEMS 

develops  in  this  way  and  with  the  dying  off  of  the  old  stem  a 
new  plant  is  established,  as  in  the  houseleek  (Fig.  70).  In  other 
cases,  runners,  almost  any  node  may  form  buds  and  roots  which 
become  separate  plants  by  the  decay  of  the  old  stem,  as  in 
the  strawberry,  gill-over- the-ground,  cinquefoil,  etc.  These  new 
plants  repeat  the  mode  of  growth  of  the  parent  plant  and  in 


FIG.  70.     Houseleek  forming  buds  at  the  end  of  short  branches  or  stolons. 

this  way  many  prostrate  stems  spread  out  over  the  soil,  estab- 
lishing new  plants  in  wider  and  wider  circles.  If  you  will  ob- 
serve the  number  of  new  plants  established  and  the  distances 
traversed  by  some  of  these  creeping  stems  each  year  you  will 
see  the  reason  for  the  common  occurrence  of  the  large  mats  and 
colonies  of  plants  with  prostrate  stems. 

48.  The  Rhizome  Type  of  Stems. — In  a  third  type,  the  stems 
have  become  so  modified  that  they  respond  to  the  various  stimuli 
in  quite  a  different  way  from  the  forms  noted  above.  They 
creep  along  in  the  soil  and  frequently  resemble  roots  more  than 
stems.  For  this  reason  they  are  called  rootstocks  or  rhizomes. 
They  are  however,  real  stems,  as  is  attested  by  the  numerous 
leaves  and  often  erect  branches  that  spring  from  their  nodes 
as  seen  in  the  ferns,  sweet  flag,  cat  tail,  grasses,  etc.  Plants  of 
this  type  are  well  protected  against  drought  and  fires,  and  like 
prostrate  stems  they  are  adapted  to  establishing  new  plants  as 
is  apparent  in  grassy  meadows,  colonies  of  golden-rod  and  daisies 
•or  reedy  banks  of  cat  tails,  sedges,  etc.  These  stems  are  also 
well  adapted  to  propagating  new  plants  because  they  generally 
serve  as  storage  organs  for  food  and  are  therefore  often  of  a 
fleshy  character  (Fig.  71).  For  these  reasons  it  is  sometimes 
very  difficult  to  eradicate  plants  of  this  type.  This  is  very  well 
illustrated  in  the  quack  grass,  often  a  troublesome  pest  in  cul- 


NATURE  OF  PLANTS  in 

tivated  land.  Plowing  and  hoeing  only  serves  to  break  up  the 
rhizome  into  numerous  parts  each  of  which  may  develop  buds 
and  roots  from  their  nodes  and  so  establish  new  plants. 

49.  The  Condensed  Type  of  Stems. — In  many  cases  we  find 
that  the  food  is  localized  in  special  regions  of  the  rhizome  which 
consequently  become  enlarged  and  rather  fleshy,  such  enlarge- 
ments appearing  at  the  end  of  the  rhizome  or  scattered  along  its 
entire  length.  Such  modified  parts  of  a  rhizome  are  called  tubers, 
g.  g.,  the  potato  and  Jerusalem  artichoke  (Fig.  72).  The  potato 


FIG.  71.  FIG.  72. 

FIG.  71.  Rhizome  of  Solomon's  seal  }wih  aerial  shoot  just  emerging  from 
the  ground.  The  seal-like  scars  mark  the  successive  shoots  produced  during 
the  past  three  years. 

FIG.  72.  Formation  of  tubers:  A,  old  potato  or  tuber  with  two  shoots 
reaching  up  into  the  air  and  from  the  base  of  these  shoots,  rhizomes  have  been 
formed  that  are  developing  new  tubers.  B,  mature  tuber  with  spirally  ar- 
ranged buds,  the  so-called  "eyes"  of  the  potato. 

is  formed  by  the  storage  of  foods  in  certain  parts  of  the  rather 
small  rhizomes  that  branch  out  from  the  stem  of  the  plant. 
The  "eyes"  are  the  buds  that  develop  at  each  node  of  the  rhizome 
and  each  is  capable  of  forming  a  shoot,  although  but  a  few  of 
them  so  function,  as  is  the  case  in  an  ordinary  branch  of  a  tree. 
By  passing  a  thread  around  a  potato  so  that  it  touches  each 
successive  bud  you  will  see  that  there  is  the  same  arrangement  of 
the  buds  as  appears  in  the  leafy  stem.  There  is  considerable 
evidence  to  show  that  the  tuber  form  of  rhizome  originated 
through  association  with  fungi.  Thus  in  the  potato  the  rhizomes 
appear  to  have  a  vigorous  normal  growth  through  the  soil  until 


ii2  CONDENSED  TYPE  OF  STEMS 

they  are  infested  with  fungi.  This  checks  their  elongation  and 
possibly  accounts  for  the  change  in  their  mode  of  growth,  which 
is  now  characterized  by  the  enlargement  of  the  infested  region 
and  the  accumulation  of  starch.  It  is  well  to  remember  that  the 
organs  of  many  of  these  plants  are  exceedingly  plastic  and  con- 
trolled to  a  remarkable  degree  by  conditions.  As  a  single  instance 
of  this  fact  note  the  leafy  stems  originating  from  the  "eyes"  of 
the  potato  are  inhibited  in  their  elongation  by  strong  light  and 
dry  air.  A  potato  grown  on  a  moist  surface  in  intense  light  and 
dry  air  develops  short  cactus  like  stems;  but  grown  in  the  dark 
and  in  moist  air  they  elongate  vigorously  after  which  their 
nature  changes,  for  if  now  exposed  to  light  they  develop  as  the 
ordinary  leafy  stem.  This  is  one  reason  for  planting  tubers 
several  inches  in  the  soil. 

A  great  variety  of  plants  develop  only  very  short  stems  that 
are  more  or  less  buried  in  the  ground.  This  type  of  stem  modi- 
fication has  many  of  the  advantages  of  the  rhizome  but  not  its 
power  to  spread  through  the  soil  and  so  spread  the  plant.  The 
vital  parts  of  such  plants  are  only  slightly  exposed  and  conse- 
quently suffer  little  from  grazing  animals  or  other  sources  of 
injury,  as  in  plantains,  dandelions,  etc.  Frequently  these  short 
stems  are  associated  with  an  abundance  of  foods  that  are  stored 
in  roots  or  other  organs.  Such  plants  can  quickly  send  up  and 
mature  their  flower  stalks  or  leafy  stems  and  thus  avoid  un- 
favorable conditions,  as  drought,  competition  with  larger  vege- 
tation that  will  appear  later,  etc.  These  facts  account  for  the 
common  occurrence  of  this  type  of  stem  in  arid  regions  where 
there  are  short  rainy  seasons.  This  habit  has  been  made  very 
conspicuous  by  cultivation  in  many  plants,  as  the  carrot,  turnip, 
radish,  and  beet.  But  in  nature  the  very  short,  almost  flat  stems 
and  fleshy  roots  are  also  of  common  occurrence,  e.  g.,  the  wild 
carrot,  wood  betony,  dandelion,  etc.  In  many  cases  the  short 
stem  itself  is  the  storage  organ  and  consequently  it  becomes  en- 
larged and  fleshy,  as  in  the  Jack-in-the-pulpit,  garden  crocus, 
spring  beauty,  etc.  (Fig.  73,  ^4).  These  short  erect  stems  are 
termed  corms  and  are  suggestive  of  the  tuber.  In  other  plants 
the  bases  of  leaves  attached  to  the  short  stems  function  for  the 


NATURE  OF  PLANTS 


storage  of  food,  as  in  the  bulb  type  of  stems  of  lilies,  onions,  etc. 
(Fig.  73,  B}. 

It  is  noteworthy  that  these  special  types  of  stems,  the  runner, 
rhizome,  bulb,  etc.,  are  the  most  common.  They  represent  the 
more  recently  evolved  forms  of  stems  and  because  of  their  ad- 
vantages they  have  become  the  dominant  forms.  The  geological 


FIG.  73.  Shortened  types  of  stems:  A,  corm  of  jack-in-the-pulpit.  At 
left  surface  view  showing  lateral  buds,  roots  and  sheathing  leaf  arising  from 
top  of  shortened  stem.  At  right  sectional  view  with  folded  leaf,  /,  in  bud  at 
apex  of  stem.  B,  bulb  type  of  shortened  stems.  At  left  bulb  of  onion  showing 
the  ensheathing  leaves  which  are  swollen  at  their  bases  with  food,  thus  forming 
the  bulb.  At  right,  section  of  a  bulb  of  hyacinth  showing  the  fleshy  leaves 
attached  to  the  very  short  stem  and  in  the  center  of  the  bulb  a  flower  cluster. 

ancestors  of  our  seed  plants  were  woody  and  tree  like.  The 
uniform  climatic  conditions  of  these  past  periods  favored  the 
tree  type  of  stem  with  its  admirable  devices  for  leaf  display. 
With  the  appearance  of  climatic  changes  upon  the  earth  there 
resulted  modifications  in  the  stem :  the  medullary  rays  tended  to 


ii4  EVOLUTION   OF   STEM   TYPES 

increase  in  size  and  so  serve  as  a  storehouse,  thus  meeting  the 
greater  need  of  reserve  material;  the  cambium  also  tended  to 
form  more  and  more  parenchymatous  tissue  so  that  the  stem 
became  less  woody  and  more  of  a  storage  organ.  So  you  can 
think  of  the  stem  becoming  reduced  in  size  and  finally  prostrate 
through  lack  of  supporting  material.  The  contact  of  the  stem 
with  the  soil  exposed  it  to  a  variety  of  new  factors  which  lead  to 
further  modifications,  some  of  which  have  survived  as  seen  in 
the  runner,  rhizome,  corm  as  well  as  in  the  annual  and  biennial 
habit  of  plants.  It  is  a  striking  fact  that  the  majority  of  our 
primitive  plants  are  woody  and  tree  like  (pines,  willows  and  oaks) 
while  the  more  recently  evolved  and  higher  types  (daisies, 
dandelions  and  golden-rods)  are  characterized  by  various  forms 
of  these  modified  stems. 


CHAPTER   IV 


THE  FLOWER,  FRUIT  AND  SEEDLING 


50.  The  Structure  of  the  Flower. — We  now  come  to  the  most 
interesting  feature  in  the  nature  and  life  of  the  plant.  All  the 
energy  of  the  plant  is  directed  towards  the  perpetuation  of  its 
kind.  Numerous  examples  have  been  noted  where  this  is  ac- 
complished by  bulbs,  rootstocks,  buds,  etc.  Among  our  common 
plants,  however,  new  individuals  are  usually  produced  from  seeds 
that  are  formed  in  the  flowers.  We  will  first  examine  the  struc- 
ture of  a  flower  and  see  how  it  is  adapted  to  the  performance  of 
this  important  work.  A  flower  is  a  highly  modified  stem  of  the 
bud  type.  This  is  very  noticeable  before  the  flower  opens  or  is 
in  bloom  (Fig.  74,  A),  A  series  of  leaves  usually  of  a  greenish 
color  envelop  the  delicate  parts  within  and  protect  them,  as  in  the 
case  of  the  bud,  against  drying  winds.  These  green  leaves  are 
known  as  the  calyx  and  each  individual  leaf  is  called  a  sepal.  As 
the  bud  opens  a  number  of  organs  are  disclosed,  particularly 
noticeable  are  a  set  of  more  delicate  and  variously  colored  leaves, 
the  corolla,  each  leaf  of  which  is  called  a  petal  (Fig.  74,  B).  The 
corolla  is  of  service  to  the  flower  in  many  ways.  Like  the  calyx 
it  may  assist  in  protecting  the  other  organs.  Especially  is  it  of 
importance  in  guarding  them  against  dews  and  rains.  For  this 
reason  the  calyx  and  corolla,  collectively  called  the  floral  envelop 
or  perianth,  in  many  flowers  often  close  at  night  or  on  rainy 
days  or  the  perianth  may  be  so  developed  or  inclined  as  to  pre- 
vent the  rain  from  falling  into  it.  It  will  be  seen  later  in  the 
work  that  the  perianth  is  of  the  greatest  importance  in  assisting 
insects  to  visit  the  flowers  in  such  a  way  that  seeds  may  be  pro- 
duced. Within  the  perianth  are  two  kinds  of  peculiar  organs, 
the  central  flask-like  ones  are  called  pistils  or,  if  united,  carpels 
and  surrounding  them  are  the  stamens  (Fig.  74,  B).  These 
two  kinds  of  organs  are  collectively  known  as  sporophylls  since 
their  special  work  is  to  produce  certain  cells  called  spores.  The 

us 


NATURE   OF  THE   SPOROPHYLLS 


sporophylls  are  the  important  organs  of  the  flower.  The  calyx 
and  corolla  may  be  lacking  but  no  seed  can  be  formed  without 
the  sporophylls. 

51.  The  Structure  of  the  Sporophylls. — Let  us  examine  the 
structure  of  these  organs  and  see  how  they  co-operate  in  the 
formation  of  the  seed.  The  stamens  consist  of  a  lobed  sack  or 
anther  which  is  usually  supported  upon  a  stalk  or  filament  (Fig. 


FIG.  74.  Flower  of  the  stonecrop,  Sedum:  A,  bud  stage  of  flower — ca, 
calyx  showing  three  sepals.  B,  open  flower — p,  petals  of  corolla;  s,  stamens; 
c,  pistils  or  carpels.  C,  stamen,  consisting  of  a  four-lobed  anther  supported 
on  delicate  stalk  or  filament.  D,  pistil  or  carpel — o,  ovary;  s,  style  which 
terminates  in  a  small  knob  or  stigma.  E,  longitudinal  section  of  ovary  showing 
row  of  ovules  attached  to  wall  of  ovary.  F,  cross-section  of  ovary,  Ovules  in 
two  rows. 

74,  C).  If  a  young  anther  is  cut  across  four  cavities,  contain- 
ing minute  or  dust-like  grains,  the  microspores,  will  be  seen 
(Fig-  75»  -4)-  The  microspores,  also  called  pollen  spores  or 
grains,  are  minute  cells,  provided  with  a  cell  wall  often  variously 
sculptured  and  contain  a  nucleus  and  protoplasm  like  an  ordinary 
cell  (Fig.  79).  At  maturity  the  anthers  break  open  in  a  manner 


NATURE  OF  PLANTS  117 

that  varies  in  different  plants,  exposing  the  microspores  to  the 
air  (Fig.  75,  B).  The  pistil  or  carpel  is  quite  different  in  char- 
acter. At  or  near  the  top  is  a  more  or  less  modified  part,  the 
stigma,  below  which  is  an  elongated  stalk,  the  style,  that  broadens 
out  into  a  base  or  ovary.  By  longitudinal  and  transverse 
sections  of  a  pistil  we  gain  a  better  idea  of  its  real  character 
(Fig.  74,  D-F).  The  ovary  is  now  seen  to  contain  a  cavity 
from  the  walls  of  which  one  or  more  rather  spherical  bodies, 
the  ovules,  are  developed.  The  structure  of  the  ovule  is  very 
complicated.  If  a  thin  section  is  made  through  the  center  of 
one  of  them  it  will  be  seen  to  contain  usually  but  one  large  spore, 
therefore  called  the  megaspore  (Fig.  76,  mg).  This  spore  is 


A 


FIG.  75.  Structure  of  the  anther:  A,  diagram  of  the  anther  cut  across, 
showing  the  four  cavities,  sporangia  (sing,  sporangium),  filled  with  spores. 
B,  cross-section  of  a  mature  anther.  The  tissue  about  the  spores  has  broken 
down,  thus  forming  two  cavities  and  at  the  left  the  breaking  open  of  the  wall 
of  the  anther  is  shown. 

developed  in  a  mass  of  tissue,  the  nucellus,  and  the  whole  is 
surrounded  by  one  or  two  coats,  the  integument,  which  grows 
up  from  the  base  of  the  ovule  but  never  quite  surrounds  it, 
thus  leaving  a  small  opening  or  micropyle  (Fig.  76,  mi).  The 
megaspore,  unlike  the  microspore,  is  never  set  free  but  is  retained 
permanently  in  the  ovule. 

It  is  not  known  how  the  sporophylls  have  originated  but  there 
are  indications  that  they  may  have  arisen  through  the  modifica- 
tion of  leaves  for  the  purpose  of  forming  spores  instead  of  con- 
structing foods.  It  is  for  this  reason  that  these  organs  are 
called  sporophylls,  or  spore-forming  leaves,  from  phylon,  a  leaf. 
Those  forming  small  spores  are  sometimes  called  microsporo- 
phylls  instead  of  stamens  and  those  forming  large  spores  mega- 
sporophylls  instead  of  pistils  or  carpels.  Very  many  modifica- 
tions of  the  sporophylls  will  be  noted  later  in  the  work. 


n8 


GERMINATION   OF  THE  SPORES 


52.  The  Germination  of  the  Microspore  and  Megaspore. — The 

question  now  naturally  arises  as  to  the  meaning  of  these  peculiar 
structures.  The  important  part  of  the  sporophyll  is  the  spore. 
A  spore  is  a  cell  that  is  capable  of  germinating  or  growing  under 
favorable  conditions  and  thus  producing  some  kind  of  a  new 
organism,  or  we  may  say  a  new  plant.  You  are  always  to  think 
of  a  spore  as  having  this  power.  In  case  of  the  spores  under 
consideration  very  rudimentary  plants  are  produced  that  con- 
sist of  only  a  few  cells.  These  minute  plants  are  very  important, 


FIG.  76.  FIG.  77. 

FIG.  76.  Section  of  the  ovule  of  the  Canada  lily  (greatly  magnified): 
mi,  micropyle;  i,  integuments;  n,  nucellus,  here  consisting  of  a  single  layer 
of  cells,  but  it  often  forms  a  large  mass  of  cells;  mg,  megaspore  with  large 
nucleus  in  center.  This  spore  is  about  to  divide;/,  stalk  or  funiculus  which 
attaches  the  ovule  to  the  wall  of  the  ovary. 

FIG.  77.  Germination  of  the  megaspore:  A,  first  division  of  the  nucleus 
of  the  megaspore.  B,  second  stage  in  the  germination,  four  nuclei  being 
formed.  C,  final  stage  in  the  division  of  the  nuclei. 

however,  because  they  contain  the  male  and  female  cells,  or 
gametes,  so  called  because  they  unite,  thus  forming  a  new  spore, 
the  gametospore,  quite  different  from  the  micro-  and  mega- 
spores.  So  let  us  not  lose  sight  of  the  fact  that  these  apparently 
insignificant  micro-  and  mega-spores  germinate  and  produce 
minute  plants  that  are  quite  distinct  from  the  plants  that  bore 


NATURE  OF  PLANTS  119 

them.  We  shall  first  follow  the  germination  of  the  megaspore 
and  note  the  character  of  the  plant  that  develops  from  it.  As 
has  been  stated  this  spore  is  never  discharged  from  the  ovule 
within  which  its  entire  growth  occurs.  The  first  indications  of 
germination  are  seen  in  the  enlargement  of  the  spore  and  the 
division  of  its  nucleus  into  two  nuclei,  which  are  pushed  to  either 
end  of  the  spore  owing  to  the  accumulation  of  water  in  the 
central  region  of  the  spore  (Fig.  77,  ^4).  The  food  for  the  nour- 
ishment of  the  growing  megaspore  is  derived  from  the  cells  of  the 
nucellus  which  are  dissolved  and  absorbed  as  the  megaspore 
increases  in  size,  and  food  is  also  conducted  to  it  from  the  parent 
plant  by  means  of  the  funiculus.  Each  of  the  two  nuclei  at  the 
ends  of  the  spore  divides  again,  thus  forming  two  nuclei  at  either 
end.  This  process  is  repeated  once  more  so  that  we  now  find  four 
nuclei  at  the  ends  of  the  spore  which  has  become  somewhat  elon- 
gated (Fig.  77,  J3,  C).  The  so-called  polar  nuclei,  a  nucleus 
from  each  end,  now  move  toward  the  center  of  the  spore  and 
fuse,  forming  one  nucleus.  This  completes  the  germination  of 
the  megaspore  and  we  see  that  it  has  grown  into  a  sac-like  plant 
consisting  of  seven  cells,  or  nuclei  imbedded  in  a  mass  of  proto- 
plasm (Fig.  78).  The  cells  of  this  minute  plant  differ  in  char- 
acter and  perform  very  different  duties.  For  example,  the  larger 
cell  toward  the  micropylar  end  of  the  plant  is  the  female  cell  or 
egg  cell.  It  will  be  called  the  female  gamete  because  it  unites 
with  another  cell,  the  male  gamete.  The  two  small  cells  near  the 
female  gamete  are  the  synergidae  or  cells  that  assist  in  the 
nourishment  and  later  development  of  the  female  gamete.  The 
two  cells  fusing  in  the  center  of  the  plant  form  the  endosperm 
nucleus  which  later  by  repeated  divisions  produces  a  mass  of 
nourishing  cells  that  fill  the  space  within  the  sac.  The  three 
cells  at  the  bottom  of  the  sac,  known  as  the  antipodal  cells,  may 
increase  in  number  and  take  part  in  the  later  growth,  but  as  a 
rule  they  are  disorganized  and  used  for  food.  They  are  often 
provided  with  cell  walls,  the  other  cells  consisting  only  of  a 
nucleus  and  a  varying  amount  of  cytoplasm.  This  plant  formed 
by  the  germination  and  growth  of  the  megaspore  is  called  the 
female  gametophyte,  because  it  contains  the  female  gamete,  and 


120         GERMINATION   OF  THE   MICROSPORE 

you  will  see  that  a  corresponding  plant  appears  in  the  ferns  and 
other  groups  of  plants. 

Returning  now  to  the  microspore,  we  find  that  its  germination 
also  often  begins  while  in  its  sporangium  but  in  other  cases 
growth  begins  after  it  has  escaped  from  the  sporangium  and  has 


FIG.  78.  The  germination  of  the  megaspore  complete.  The  plant  thus 
formed  consists  of  a  female  gamete  or  egg  cell,  9  ,  below  which  are  two  syner- 
gidae  and  in  the  center  are  the  two  uniting  polar  nuclei,  p,  while  at  the  opposite 
end  of  the  sac  are  three  antipodal  cells,  a;  mi,  micropyle;  i,  integuments;  /, 
stalk  or  funiculus  in  which  a  vascular  bundle,  v,  has  been  developed  to  trans- 
port food  from  the  plant  to  the  ovule. 

been  carried  by  the  wind  or  some  insect  visiting  the  flower  for 
food,  to  the  stigma  of  the  pistil.  The  stigma  is  admirably 
adapted  to  hold  the  microspores,  being  provided  with  minute  out- 
growths, and  frequently  sugary  solutions  are  exuded  which  fasten 
the  spores  to  the  stigma.  The  real  importance  of  these  sugary 


NATURE   OF   PLANTS  121 

solutions  is  to  nourish  the  microspores.  It  is  evident  that  these 
minute  dust-like  cells  can  not  contain  sufficient  food  to  bring 
about  any  considerable  growth.  It  is  noteworthy  that  the 
strength  and  nature  of  the  sugar  solutions  varies  in  different, 
stigmas  and  that  the  microspores  are  often  able  to  grow  only 
in  solutions  of  such  strength  or  in  the  presence  of  such  substances 
as  are  found  on  the  stigmas  of  their  own  kind  of  plant.  So  it 
results  that  the  microspores  are  often  unable  to  grow  when 
carried  to  the  stigmas  of  a  different  kind  of  plant  from  them- 
selves. The  germination  of  the  microspore  results  in  the  division 
of  its  nucleus  and  the  ormation  of  a  large  and  a  small  cell  known 
as  the  tube  and  antheridial  cell  respectively  (Fig.  79,  A,  B). 
The  latter  cell  divides  once,  forming  two  cells,  called  the  male 
gametes  (Fig.  79,  C).  This  stage  in  the  germination  of  the 
microspore  is  reached  either  in  its  sporangium  or  after  being 
transferred  to  the  stigma.  Nourished  by  the  foods  of  the  stigma 
the  microspore  continues  its  growth,  the  tube  cell  ruptures 
the  outer  wall  of  the  spore  and  forms  a  tube-like  growth.  This 
tube  cell  grows  down  between  the  cells  of  the  style  into  the 
cavity  of  the  ovary  where  it  usually  curves  out  and  enters  the 
ovule  by  way  of  the  micropyle.  It  now  works  its  way  through 
the  tissues  of  the  nucellus  to  the  female  gametophyte,  into  the 
cavity  of  which  it  enters  by  dissolving  the  wall.  In  the  mean- 
time the  two  male  gametes  have  been  carried  down  by  the 
streaming  movement  of  the  cytoplasm  to  the  end  of  the  tube 
cell  as  it  enters  the  female  gametophyte  (Fig.  79,  D).  The 
question  will  naturally  be  asked,  what  directs  the  peculiar  growth 
of  this  minute  plant?  In  the  first  place  the  tube  cell  is  repelled 
by  the  oxygen  of  the  atmosphere,  so  as  soon  as  it  appears,  it  is 
directed  away  from  the  atmosphere  into  the  stigma.  Its  course 
down  the  style  to  the  ovary  is  very  largely  controlled  by  lines  of 
loose  tissue  through  which  it  can  easily  work  its  way  and  also 
by  foods  which  are  deposited  in  the  cells  just  ahead  of  the  tube 
cell  so  that  it  is  led  along  rather  straight  pathways  to  the  ovary. 
The  curving  out  of  the  tube  to  the  micropyle  and  its  subsequent 
growth  down  into  the  female  gametophyte  can  only  be  accounted 
for  by  some  chemical  stimulus  that  is  located  in  the  ovule  and 
9 


122 


FECUNDATION   OR   FERTILIZATION 


female  gametophyte.  A  similar  attraction  brings  about  a  move- 
ment of  the  cytoplasm  which  carries  the  male  gametes  to  the 
end  of  the  tube  cell.  It  will  be  seen  that  this  peculiar  plant 
(Fig.  79,  D)  consisting  of  a  long  tubular  growth  with  three 
naked  cells  is  more  rudimentary  than  the  plant  derived  from  the 
megaspore.  Because  this  plant  produces  the  male  cells  or  male 


FIG.  79.  Germination  of  the  microspore:  A,  mature  microspore.  B,  first 
stage  in  its  germination — t,  tube  cell;  a,  antheridial  cell.  C,  end  of  cell  divi- 
sion— t,  tube  cell;  a,  antheridial  cell  forming  two  male  gametes.  D,  diagram 
showing  the  formation  of  the  tube  which  grows  through  the  style  and  finally 
reaches  the  female  gametophyte.  The  two  male  gametes,  g,  are  shown  passing 
down  the  tube;  t,  tube  nucleus. 

gametes  we  call  it  the  male  gametophyte.  So  we  see  that  the 
flower  forms  two  kinds  of  spores  and  that  these  germinate  and 
form  two  kinds  of  plants,  a  male  and  a  female. 

53.  Fecundation  or  Fertilization. — As  soon  as  the  develop- 
ment of  these  two  plants  is  completed  a  very  remarkable  change 
takes  place  which  starts  an  entirely  new  growth  that  is  quite 
independent  of  the  development  of  the  male  and  female  gameto- 
phyte. The  end  of  the  tube  cell  becomes  distended  through  the 
accumulation  of  material  and  finally  ruptures,  discharging  the 
male  gametes  into  the  female  gametophyte.  The  chemical  com- 
position of  the  female  gamete  is  such  that  one  of  the  male  cells 
is  attracted  to  it  (Fig.  80)  and  finally  the  two  gametes  unite 
forming  a  single  cell.  This  body  is  a  sexually  formed  spore  or 
gametospore  and  like  the  ordinary  spore  has  the  power  to  germi- 


NATURE  OF   PLANTS 


123 


nate  and  produce  a  plant.  The  microspores  and  megaspores, 
however,  were  not  formed  by  the  union  of  sexual  cells  or  gametes. 
Therefore  we  may  distinguish  them  as  asexual  spores  or  simply 
spores.  It  has  frequently  been  observed  that  the  endosperm 
nucleus  also  attracts  the  other  male  gamete  and  causes  a  similar 
fusion  with  it  (Fig.  80).  The  union  of  the  male  and  female 


FIG.  80.  The  micropylar  end  of  an  ovule  of  Canada  lily  (sectional  view), 
showing  the  process  of  fertilization  or  fecundation.  The  tube,  t,  has  grown 
into  the  female  gametophyte  and  ruptured,  discharging  the  two  male  gametes. 
One,  cT,  is  seen  fusing  with  the  female  gamete,  9,  and  the  other  one,  c?',  is 
uniting  with  the  two  polar  cells,  thus  forming  the  endosperm  nucleus;  s,  one 
of  the  synergids;  «,  integuments.  *• 

gametes  is  called  fertilization  or  fecundation.  Unless  fertilization 
is  effected  the  growth  outlined  above  usually  ends  the  history  of 
the  flower.  But  if  fecundation  is  effected  then  a  gametospore 
is  formed  that  is  capable  of  germinating  and  producing  a  plant 
whose  growth  and  development  are  attained  with  most  remark- 
able changes  of  the  ovule  and  often  of  the  surrounding  parts. 
54.  The  Germination  of  the  Gametospore  and  the  Formation 
of  Seed. — The  first  indication  of  the  germination  and  growth  of 
the  gametospore  is  seen  directly  after  fertilization.  It  becomes 


124        GERMINATION   OF  THE   GAMETOSPORE 

surrounded  by  a  cell  wall  and  attached  to  one  side  of  the  female 
gametophyte,  which  is  henceforth  called  the  embryo  sac  (Fig. 
81,  A).  By  a  series  of  divisions  the  gametospore  now  forms  a 


FIG.  81.  Germination  of  the  gametospore  of  peppergrass,  Lepidium:  A: 
micropylar  end  of  the  embryo  sac  showing  that  the  gametospore,  g  (fertilized 
female  gamete),  has  developed  a  cell  wall  and  become  attached  to  the  wall 
of  the  embryo  sac.  Compare  Fig.  80.  B,  later  stage  in  the  germination — e, 
embryo  cell;  s,  suspensory  cells;  en,  endosperm  cells.  C,  the  embryo  cell 
has  formed  two  cells.  D,  later  stage.  The  embryo  cell  by  further  division 
has  formed  a  spherical  mass  of  cells,  here  shown  in  section.  Note  the  appear- 
ance of  an  outer  layer  of  cells,  the  epidermis,  and  a  central  or  stem  region, 
s,  a  few  of  the  suspensory  cells.  E,  still  later  growth.  Two  growing  .regions, 
the  cotyledons  are  appearing  on  the  side  of  the  stem.  F,  the  micropylar  end  of 
the  embryo  sac,  showing  the  stage  of  development  where  the  parts  of  a  small 
plant  can  be  clearly  recognized — c,  cotyledons;  st,  stem  ending  in  root,  r, 
to  which  is  still  attached  the  supensory  cells;  en,  endosperm  cells,  now  pro- 
vided with  walls,  are  being  absorbed  as  the  plant  or  embryo  enlarges.  Note 
that  the  embryo  and  embryo  sac  are  slightly  bent  to  the  left.  This  curvature 
is  due  to  the  fact  that  the  ovule  in  a  great  many  plants  becomes  curved  in 
its  development  with  the  result  that  the  embryo  sac  assumes  a  curved  or 
U-shaped  form.  This  form  of  the  ovule  results  in  a  complete  bending  over 
of  the  cotyledons  against  the  stem,  as  shown  in  the  next  figure. 


NATURE   OF   PLANTS  125 

number  of  more  or  less  elongated  cells,  the  so-called  suspensor, 
and  a  terminal  cell  or  embryo  cell  (Fig.  81,  B,  V).  In  the 
meantime  the  endosperm  cell  has  divided  again  and  again  and 
the  resulting  naked  nuclei  have  become  arranged  around  the 
sides  of  the  embryo  sac  (Fig.  81,  B,  en).  This  formation  of 
endosperm  cells  continues  until  they  more  or  less  fill  the  embryo 
sac  and  they  also  become  surrounded  with  cell  walls.  These 
endosperm  cells  are  filled  with  food  and  serve  to  nourish  the 
germinating  gametospore  just  as  the  nucellus  nourished  the 
female  gametophyte.  While  the  suspensory  cells  usually  soon 
cease  to  grow,  the  embryo  cell  divides  very  actively,  as  shown 
in  Fig.  81,  C-E,  and  soon  there  is  a  clear  indication  of  a  minute 
plant  with  root,  stem  and  leaves  (Fig.  81,  F).  This  minute 
plant  developed  by  the  germination  of  the  gametospore,  is  called 
the  embryo.  In  the  case  of  the  plant  under  consideration  the 
embryo,  when  fully  formed  consists  of  a  stem  with  two  laterally 
placed  leaves,  the  cotyledons,  and  a  root.  The  region  of  the 
stem  above  the  attachment  of  the  cotyledons  is  known  as  the 
plumule  and  frequently  assumes  the  form  of  a  minute  bud.  The 
region  of  the  stem  below  the  cotyledons  is  termed  the  hypocotyl 
and  the  root  and  root  cap  appear  at  its  lower  end  (Fig.  82,  A). 
The  ovule  containing  the  mature  embryo  is  called  a  seed.  The 
interesting  feature  about  the  seed  is  the  fact  that  it  is  a  structure 
in  which  growth  has  ceased  and  that  it  is  capable,  owing  to  a 
remarkable  series  of  devices  that  will  be  noted  directly,  of  re- 
maining in  this  condition,  i.  e.,  dormant  often  for  one  or  more 
years. 

Great  variation  characterizes  the  germination  of  the  gameto- 
spore in  the  different  groups  of  plants,  but  the  development  out- 
lined above  is  fairly  characteristic  of  those  plants  that  form  an 
embryo  with  two  cotyledons.  Such  plants  are  called  for  this 
reason  dicotyledons.  Other  plants  develop  an  embryo  with  but 
one  cotyledon,  as  our  grasses,  lilies,  etc.,  and  for  this  reason  they 
are  called  monocotyledons.  Further  variations  will  be  seen  in 
cone-bearing  trees,  ferns,  etc. 

It  must  be  borne  in  mind  that  the  formation  of  the  seed  is 
also  attended  with  profound  changes  in  the  structure  of  the 


126 


STRUCTURE   OF  THE  SEED 


ovule,  and  often  in  the  pistil  and  surrounding  parts.  Frequently 
the  endosperm  grows  so  extensively  as  to  absorb  and  replace  the 
cells  of  the  nucellus  and  thus  comes  to  occupy  all  the  space  within 
the  coats  of  the  integument,  as  in  the  castor  bean,  morning  glory, 
onion,  etc.  (Fig.  85,  B}.  The  embryo  may  remain  comparatively 


FIG.  82.  Structure  of  the  seed:  A,  section  of  an  ovule  of  peppergrass  in 
which  the  growth  of  the  embryo  is  nearly  complete — -ft  stalk  or  funiculus 
attaching  ovule  to  wall  of  ovary;  mi,  micropyle;  i,  integument;  en,  remains 
of  endosperm.  The  embryo  consists  of  a  stem  which  is  differentiated  into 
a  hypocotyl,  hy,  that  ends  below  in  a  root,  r,  and  root  cap,  and  above  in 
the  plumule,  pi,  and  two  cotyledons,  c,  which  curve  over  and  lie  one  upon  the 
other,  v,  vascular  bundles  which  extend  up  through  the  stem  into  the  coty- 
ledons where  they  subdivide,  forming  a  network  of  veins.  B,  section  of  seed 
of  water  lily,  after  Conard — e,  embryo  surrounded  by  a  layer  of  endosperm 
cells;  mg,  cells  of  the  nucellus;  i,  integument. 

small  as  in  the  cases  just  cited  or  it  may  in  turn  absorb  and 
replace  all  the  cells  of  the  endosperm  and  so  come  to  occupy 
the  space  within  the  integument,  as  in  the  bean,  pea,  etc.  (Figs. 
82,  A]  83,  J9).  Various  other  arrangements  may  result;  as  for 
example,  there  may  be  remains  of  the  nucellus  and  endosperm 
with  the  embryo  (Fig.  82,  B).  The  integument  also  undergoes 
various  changes  during  these  growths,  often  becoming  hard  or  pa- 


NATURE   OF   PLANTS  127 

pery  or  provided  with  hairs,  hooks,  spines,  and  smooth  or  pitted. 
Compare  the  bean,  catalpa,  cotton,  milkweed,  etc.  Attention 
must  also  be  directed  to  the  growths  that  occur  outside  of  the 
seed  that  are  induced  by  fertilization  and  the  germination  of  the 
gametospore.  Very  frequently  the  pistil  becomes  tough  and 
hard  and  invests  the  seed  like  a  second  integument,  as  in  wheat, 
corn,  acorn,  etc. ;  again  it  forms  a  sac  about  the  seed  as  in  the 
sedges  and  buttercups;  or  it  may  enlarge,  forming  the  pods  of 
peas  and  beans  or  the  capsules  of  lilies.  In  other  cases  it  be- 
comes fleshy  throughout  as  in  the  tomato  and  currant,  or  only 
the  outer  walls  are  fleshy  while  the  inner  part  forms  the  pit  or 
stone  as  in  the  peach  and  plum.  Frequently  wings  and  hooks 
grow  out  from  the  pistil  as  in  the  maple,  Ailanthus,  etc.  Even 
the  end  of  the  stem,  the  receptacle,  which  bears  the  various 
organs  of  the  flower,  may  be  induced  to  grow,  as  in  the  apple, 
where  it  surrounds  the  pistils  (the  core)  with  a  fleshy  coat,  or 
the  receptacle  may  enlarge,  as  in  the  strawberry,  forming  the 
fleshy  part  of  the  fruit.  In  other  cases  outgrowths  below  the 
flower  develop,  forming  the  burr  of  the  chestnut,  clotbur,  beech- 
nut, and  the  cup  of  the  acorn.  The  entire  growth  that  is  asso- 
ciated with  the  formation  of  the  seed  is  termed  the  fruit  and  we 
see  that  it  may  include  a  variety  of  modified  organs.  The  term 
seed,  however,  refers  only  to  the  modified  ovule  and  its  embryo. 
Later  in  the  work  it  will  be  noted  that  these  growths  are  often 
devices  to  bring  about  a  distribution  of  the  seed  or  to  protect  it 
during  its  dormant  period. 

55.  The  Seed  and  its  Growth. — Spores  of  all  kinds  germinate 
and  produce  new  plants.  Seeds  do  not  germinate.  They  simply 
continue  the  growth  of  the  embryo  or  plant  which  has  been  tem- 
porarily stopped.  The  falsely  called  germination  of  the  seed  is 
but  the  awakening  or  renewal  of  the  growth  of  this  plant.  The 
seed  is  essentially  a  young  plant  that  is  supplied  with  a  certain 
amount  of  food  and  that  has  temporarily  stopped  growing.  The 
seed  may  be  looked  upon  as  one  of  the  many  adaptations  found 
among  plants  to  ensure  their  distribution  and  perpetuation. 
From  a  seed  a  single  plant  is  developed  that  may  produce  many, 
often  thousands  of  seeds,  i.  e.,  plantlets.  We  have  already  noted 


128  STRUCTURE   OF  THE   SEED 

that  these  may  be  widely  distributed  and  so  spread  and  multiply 
the  number  of  plants.  Some  seeds  are  so  fine  that  they  float  away 
as  a  dust,  as  in  the  orchids.  Other  seeds  and  fruits  are  provided 
with  hairs  and  wings  that  serve  to  buoy  them  up  or  promote 
their  transport  by  winds,  as  in  the  case  of  pine  seeds,  milk- 
weed, willow,  and  the  fruits  of  maple,  dandelion,  etc.  In  other 
cases  spines  and  hooks  or  mucilage  glands  are  developed  that 
attach  them  to  any  passing  animal  and  so  bring  about  a  con- 
siderable distribution.  Note  the  common  devices  of  this  nature, 
as  in  the  burdock,  stick  tight,  beggar  lice,  agrimony  or  the  glands 
on  the  nightshade  (Circaea)  and  twin  flower.  So  also  seeds 
are  scattered  by  the  spring  of  elastic  stems  and  explosive  fruits, 
as  in  many  lilies,  witch  hazel,  violet,  touch-me-not,  etc.  But 
especially  may  seeds  be  looked  upon  as  an  adaptation  to  tide 
the  plant  over  seasons  unfavorable  for  growth.  In  this  respect 
they  may  be  compared  to  buds,  which  they  also  resemble  in  their 
renewal  of  growth  when  conditions  are  again  favorable.  There 
are  some  noteworthy  exceptions  to  this  statement,  as  in  the  man- 
grove, certain  oaks,  and  many  grasses.  The  seeds  of  the  former 
plant  begin  their  growth  while  still  attached  to  the  tree  and  a 
similar  growth  has  also  been  observed  in  the  acorns  of  the 
white  oak.  Grain  often  sprouts  while  in  the  sheaves  and  the 
seeds  from  green  tomatoes  produce  earlier  and  larger  fruits. 
However,  the  majority  of  seeds  and  fruits,  as  well  as  buds,  do 
not  renew  their  growth  immediately  after  their  formation.  A 
longer  or  shorter  resting  or  dormant  period  is  required  during 
which  time  chemical  changes  occur  that  render  the  foods  soluble 
and  therefore  available  to  the  young  plant  or  embryo.  For  ex- 
ample, in  many  seeds  and  buds  the  storage  foods  are  in  the  form 
of  starch.  This  food  is  insoluble  and  can  not  be  used  by  the 
plant  until  it  has  been  acted  upon  by  an  enzyme  which  changes 
it  into  a  soluble  and  readily  diffusible  sugar.  These  enzymes  are 
slowly  formed  during  the  resting  period  and  a  renewal  of  growth 
is  not  possible  until  they  have  begun  the  transformation  of  the 
insoluble  foods  into  soluble  forms.  The  changes  thus  effected 
are  so  considerable  that  growing  tubers,  rootstock  and  seeds 
often  have  a  sweet  taste  owing  to  the  considerable  amount  of 


NATURE   OF   PLANTS  129 

sugar  present.  So  the  seed  may  be  looked  upon  as  a  variation 
in  the  growth  of  the  plant  that  enables  it  to  multiply  and  dis- 
tribute its  kind  and  to  meet  the  various  problems  in  connection 
with  its  existence,  such  as  drought  and  sometimes  even  fires. 

56.  Conditions  Necessary  for  Growth. — The  external  condi- 
tions necessary  for  starting  the  growth  of  the  embryo  are  a 
suitable  temperature,  moisture,  and  the  oxygen  of  the  atmos- 
phere. Light  only  in  rare  instances  appears  to  be  a  contributing' 
factor  in  the  renewal  of  growth.  Some  seeds  will  sprout  at  near 
the  freezing  point,  while  others  require  a  temperature  between 
10°  and  17°  C.  It  is  interesting  to  note  that  the  rising  tempera- 
ture together  with  other  factors  as  the  season  advances  from 
early  spring  bring  out  a  regular  succession  in  the  awakening  of 
various  kinds  of  seeds.  A  suitable  amount  of  moisture  must 
also  be  supplied  to  the  seed  to  assist  in  the  solution  and  diffusion 
of  the  foods  to  the  embryo.  This  is  very  necessary  since  the 
seed  contains  very  little  water.  In  fact  the  seed  owes  its  vitality 
and  power  to  resist  long  drought  and  severe  cold  to  its  dry  con- 
dition. Seeds  soaked  in  water  cannot  endure  such  extremes  as  in 
a  dry  state.  Excess  of  water,  however,  is  unfavorable  to  the 
renewal  of  growth,  because  it  excludes  the  oxygen  of  the  air. 
This  is  the  cause  of  the  failure  to  grow  and  of  the  decay  of  the 
seeds  planted  in  the  spring  when  the  season  is  rainy.  The  air 
spaces  in  the  soil  become  filled  with  water  and  as  a  consequence 
oxygen  can  not  gain  access  to  the  seed  and  co-operate  in  those 
chemical  changes  that  not  only  supply  some  of  the  foods  to  the 
embryo  but  also  provide  it  with  the  energy  for  growth.  An 
examination  of  a  few  seeds  will  show  some  of  the  more  important 
structural  features  that  adapt  them  to  the  conditions  that  they 
have  to  meet. 

In  the  case  of  the  bean  the  integument  is  tough  and  would 
appear  to  offer  a  rather  effectual  barrier  to  the  entrance  of  water 
and  air.  You  have  noted  that  a  minute  opening,  the  micro- 
pyle,  exists  in  the  integument.  This  opening  is  seen  near  the 
scar  or  hilum  that  marks  the  point  where  the  seed  was  attached 
by  a  minute  stem  to  the  walls  of  the  ovary  (Fig.  83,  B).  This 
opening  permits  the  entrance  of  the  air  and  water  as  does  the 


130 


NATURE   OF  THE   SEED 


looser  structure  of  the  hilum.  This  can  easily  be  demonstrated 
by  placing  beans  in  water,  when  the  spread  of  the  water  from  the 
region  of  the  micropyle  will  be  indicated  by  the  swelling  and 
wrinkling  of  the  integument  (Fig.  83,  C).  After  a  time  the 
entire  integument  becomes  quite  soft  and  water  is  readily  drawn 
through  it  owing  to  the  fact  that  the  storage  foods  in  the  embryo 
are  in  close  contact  with  it  and  draw  the  water  through  by 
'osmosis.  Carefully  removing  the  integument  from  a  soaked  bean 


FIG.  83.  Fruit  and  seed  of  the  bean:  A,  mature  fruit  or  pod:  s,  style; 
c,  remains  of  calyx;  0,  ovary.  At  left  the  pod  has  been  opened,  showing  four 
seeds  attached  to  the  walls  of  the  ovary  by  funiculus.  B,  two  views  of  a 
seed — h,  hilum  or  scar  marking  attachment  of  funiculus  to  seed;  m,  micropyle. 
C,  wrinkling  of  seed  coat  caused  by  the  entrance  of  water  through  the  micro- 
pyle and  hilum.  D,  embryo  of  seed  after  removal  of  integument — c,  coty- 
ledons. At  right  one  of  the  cotyledons  has  been  removed  to  show  the  plumule, 
ep,  and  the  hypocotyl,  hy.  E,  half  of  the  membranous  integument. 

we  note  that  the  embryo  is  well  developed,  consisting  of  two 
fleshy  cotyledons  and  a  short  stem  (Fig.  83,  D,  E).  The  plumule 
is  well  developed,  bearing  two  leaves,  and  the  short  hypocotyl 
and  root  are  enclosed  in  a  sheath  formed  from  the  integument. 
Note  that  the  hypocotyl  and  root  are  situated  near  the  micropyle. 
How  does  this  arrangement  work  to  the  advantage  of  the  em- 
bryo? Allow  the  seed  to  germinate  and  it  will  be  noted  that  the 
absorption  of  water  first  causes  the  embryo  to  expand  and  rupture 
the  integument.  This  is  immediately  followed  by  the  growth  and 
elongation  of  the  lower  part  of  the  hypocotyl  which  results  in  the 
pushing  out  of  the  root  (Fig.  84,  A).  Thus  it  happens  that  the 
very  part  of  the  embryo  that  shows  the  first  signs  of  growth  is  in 


NATURE  OF   PLANTS 


close  proximity  to  the  water  as  it  enters  the  seed  and  doubtless 
the  sheath  also  assists  in  retaining  water  in  this  region.  We  are 
already  familiar  with  the  properties  of  the  root  which  direct  its 
growth  in  such  a  way  that  no  matter  how  the  seed  may  be  placed 
the  root  will  ultimately  grow  into  the  soil  and  toward  water  and 
the  soil  foods,  page  61.  It  is  noteworthy  that  many  seeds  are  so 
fashioned  that  they  naturally  assume  such  a  position  on  the 
ground  as  to  bring  the  root  as  it  emerges  into  direct  contact  with 
the  soil.  Such  an  arrangement  is  equally  advantageous  for  the 
absorption  of  water.  How  many  seeds  can  you  find  that  will 
naturally  lie  upon  the  soil  in  such  a  way  as  to  cause  the  root  on 


FIG.  84.  Renewal  of  growth  of  the  bean  seed:  A,  basal  region  of  hypocotyl 
elongating  and  pushing  the  root  into  the  soil.  B,  upper  region  of  the  hypo- 
cotyl elongating,  lifting  the  cotyledons  and  epicotyl  above  the  soil.  C,  hypo- 
cotyl erect  and  epicotyl  expanding,  forming  the  first  normal  leaves. 

emerging  to  be  directed  away  from  the  earth?  Thus  at  the  very 
start  the  growth  of  the  seed  is  of  such  a  character  that  the  young 
plant  is  anchored  to  the  soil  by  the  root  and  brought  into  proper 
relation  with  the  water  and  other  materials  necessary  for  the 
construction  of  foods.  As  soon  as  this  work  is  well  under  way 
the  upper  end  of  the  hypocotyl  begins  to  elongate,  pushing  up 
into  the  air  cotyledons  and  plumule  (Fig.  84,  B).  Here  we 


132  GROWTH   OF  THE  SEED 

see  that  the  upper  end  of  the  hypocotyl  reacts  to  gravity  like  a 
stem  and  consequently  grows  away  from  the  earth.  This  growth 
is  attended  with  a  curving  and  constant  nutation  of  the  hypo- 
cotyl which  enables  it  to  work  its  way  through  the  ground  to 
better  advantage  than  would  be  the  case  if  it  were  pushed  pas- 
sively up.  Darwin  compares  this  work  of  the  seedling  in  emerg- 
ing from  the  soil  to  the  struggles  of  a  man  who  is  prostrate  and 
attempting  to  arise  with  a  heavy  load  on  his  back.  The  ability 
of  the  hypocotyl  to  overcome  obstacles  in  its  way  and  crowd 
through  small  openings  is  due  to  the  power  generated  in  the 
enlarging  cells.  In  this  way  a  force  is  often  created  that  may 
exceed  80  pounds  to  the  square  inch,  over  five  times  the  atmos- 
pheric pressure.  In  many  seeds  only  the  region  above  the  coty- 
ledons elongates,  leaving  the  cotyledons  in  the  ground,  as  in  the 
pea,  acorns,  walnuts,  etc.  The  elongating  region  is  called  the 
epicotyl.  This  behavior  is  doubtless  associated  with  the  storage 
of  food.  In  some  seeds  the  cotyledons  serve  only  as  storage  or- 
gans and  in  such  cases  the  position  of  advantage  is  in  the  soil 
where  they  are  better  protected  and  in  contact  with  moisture 
which  will  assist  in  the  solution  and  diffusion  of  the  foods.  In 
many  cases  the  cotyledons  perform  a  dual  function,  first  as  storage 
organs  and  later  expanding  and  developing  into  the  first  green 
leaves  of  the  plant,  as  in  the  squash  and  many  of  our  common 
plants.  In  any  of  these  cases  it  is  evident  that  the  young  plant 
requires  a  certain  amount  of  food  to  enable  it  to  get  established 
in  the  soil.  It  is  not  until  considerable  growth  has  taken  place 
that  it  develops  its  first  green  leaves  and  so  begins  to  be  self 
supporting.  In  the  case  of  the  bean  and  in  many  other  seeds  and 
fruits  this  need  is  amply  met  by  the  abundance  of  food  that  is 
stored  in  the  fleshy  cotyledons  (Fig.  84,  C).  These  organs  re- 
main attached  to  the  plant  even  after  it  is  well  established,  and  it 
is  only  after  the  food  has  been  withdrawn  that  the  shriveled 
cotyledons  drop  off.  As  the  young  plant  becomes  established 
the  plumule  unfolds  like  a  bud  and  develops  the  stem  and  leaves 
and  finally  the  flowers  and  fruit  with  which  you  are  familiar. 

The  castor  bean  shows  quite  a  variation  from  the  structure 
of  the  bean.     The  hilum  and  micropyle  are  more  or  less  con- 


NATURE  OF   PLANTS 


133 


cealed  by  a  fleshy  outgrowth,  the  caruncle  (Fig.  85,  A).  Remov- 
ing the  hard  integument  we  find  within  a  white,  oily  mass  of 
cells  which  are  attached  to  the  horn-like  coat  only  at  the  hilum. 
Cutting  across  this  mass  of  cells  it  will  be  seen  that  two  thin 
leaves  are  imbedded  in  the  center.  Taking  two  other  seeds  and 
cutting  them  longitudinally,  one  at  right  angles  and  the  other  par- 
allel to  the  leaves  we  see  that  the  embryo  consists  of  a  very  small 
stem  and  root  and  two  delicate  leavesimbedded  in  an  oily  tissue, 
the  endosperm  (Fig.  85,  B).  In  the  bean  the  seed  consisted  of 


Fib.  85.  Seed  and  seedling  of  the  castor  bean:  A,  the  seed  with  fleshy 
outgrowth,  the  caruncle,  c,  at  base.  B,  section  of  seed.  At  right  showing 
the  two  cotyledons,  c,  and  short  hypocotyl  in  center  of  the  endosperm,  en. 
At  left  the  seed  is  split  parallel  to  the  surface  of  the  delicate  white  cotyledon. 
C,  early  stage  in  the  growth  of  the  seedling.  The  hard  integument  is  being 
thrown  off  but  the  cotyledons  are  still  entirely  buried  in  the  endosperm.  D, 
later  stage.  The  cotyledons,  c,  still  absorbing  food  from  the  endosperm,  en, 
are  beginning  to  appear  and  to  develop  chlorophyll.  E,  the  cotyledons  and 
plumule  are  expanding,  the  endosperm  being  entirely  absorbed. 

an  embryo  and  integument.  In  the  castor  bean,  the  endosperm 
forms  the  bulk  of  the  seed,  since  it  has  been  only  partially  con- 
sumed by  the  embryo.  It  would  naturally  be  inferred  that  the 
hard  coat  would  prevent  the  access  of  water  and  gases.  Observe, 


134  GROWTH   OF  THE   CASTOR   BEAN 

however,  that  the  caruncle  readily  absorbs  water  and  that  the 
embryo  and  endosperm  are  in  direct  contact  with  this  region 
of  the  seed.  Many  devices  will  be  observed  among  seeds  serv- 
ing to  convey  water  to  the  growing  parts.  Note  the  mucilag- 
inous coats  on  flax,  quince,  many  mustard  seeds,  etc.,  or  the 
spongy  rinds  of  the  fruits  of  the  walnut,  cocoanut,  etc.  It  is 
well  worth  your  time  to  submerge  in  water  colored  a  deep  red 
with  eosin  widely  different  seeds  and  examine  them  from  time 
to  time,  noting  the  period  required  for  the  penetration  of  water 
and  the  lines  along,  which  the  water  enters  and  is  distributed. 
It  will  be  found  that  many  seeds  and  fruits  exclude  water  and 
oxygen  very  effectively  for  one  or  more  years  with  the  result 
that  the  renewal  of  growth  is  delayed  until  the  decay  of  the 
parts  permits  their  entrance.  The  breaking  of  the  integument 
would  appear  as  a  serious  obstacle  to  the  growth  of  the  embryo. 
However,  the  power  generated  by  the  cells  owing  to  their  ex- 
pansion through  the  absorption  of  water  is  quite  sufficient  to  rup- 
ture the  seed  coat.  The  early  growth  of  the  embryo  of  the  castor 
bean  is  similar  to  that  of  the  bean.  The  elongation  of  the  lower 
end  of  the  hypocotyl  pushes  the  root  into  the  soil,  while  the 
upper  end  of  the  hypocotyl,  arching  and  twisting  as  it  elongates, 
works  the  cotyledons  and  endosperm  through  the  soil  into  the 
air  (Fig.  85,  C).  In  this  seed  the  food  is  without  the  cotyle- 
dons. Consequently,  although  the  cotyledons  gradually  enlarge 
and  become  green,  they  remain  in  contact  with  the  endosperm 
.until  all  its  food  is  absorbed  and  but  a  papery  skin  remains 
(Fig.  85,  D). 

A  grain  of  corn  will  illustrate  another  modification  of  the 
seed.  In  this  case  the  pistil  remains  in  such  close  contact  with 
the  seed  that  it  appears  to  form  an  additional  integument.  Such 
a  fruit  is  called  a  grain  and  may  be  compared  to  a  bean  pod  with 
a  single  seed.  We  can  still  see  upon  the  grain  (Fig.  86,  A} 
at  5  the  position  of  the  style  while  at  p  appears  the  remains  of 
the  stem  that  fastened  the  sporophyll  to  the  cob.  The  long 
threads,  often  called  the  silk,  that  project  from  the  ears  of  corn 
are  greatly  enlongated.  stigmas  for  catching  the  microspores. 
While  the  outer  parts  of  the  grain  of  corn  are  very  hard,  if  it 


NATURE  OF  PLANTS 


135 


is  treated  with  the  eosin  solution  referred  to  above  it  will  be 
seen  that  at  first  the  water  enters  by  way  of  the  little  stalk  just 
as  did  the  food  that  was  supplied  to  the  growing  grain.  In 
this  way  water  is  supplied  directly  to  the  root  of  the  embryo 
which  lies  near  the  stalk  as  shown  in  Fig.  86,  B.  If  two  seeds 
are  soaked  until  soft  and  then  sectioned  so  as  to  cut  one  across 
and  the  other  longitudinally  through  the  embryo  it  will  be  seen 


FIG.  86.  Grain  and  seedling  of  corn:  A,  two  views  of  a  grain — s,  point  of 
attachment  of  style;  p,  stalk  attaching  the  grain  to  cob;  emb,  location  of 
embryo.  B,  longitudinal  section  of  a  grain.  The  embryo  consists  of  a 
plumule,  pi,  root,  r,  and  scutellum,  sc.  en,  endosperm.  C,  cross-section  of 
a  grain  showing  endosperm,  en,  surrounding  the  embryo,  emb.  D,  diagram 
of  the  outer  cells  of  the  scutellum,  showing  the  elongated  cells  that  bring 
about  a  solution  and  absorption  of  the  foods  stored  in  the  endosperm.  E-G, 
stages  in  the  growth  of  the  seedling. 

that  the  endosperm  forms  the  larger  part  of  the  grain  and  that 
the  embryo  is  relatively  small,  consisting  of  a  stem  and  root,  a 
plumule  of  several  closely  wound  leaves,  and  a  shield-like  organ, 
the  scutellum  (Fig.  86,  B,  C).  The  entire  embryo  may  easily  be 
removed  from  the  endosperm  with  a  knife  if  the  seed  is  thoroughly 
soaked.  Unlike  the  two  preceding  examples  the  corn  is  a  mono- 
cotyledon and  many  regard  the  scutellum  as  a  modified  coty- 
ledon. This  organ  forms  the  enzymes  which  put  into  soluble 


136  ALTERNATION   OF   GENERATIONS 

form  the  food  in  the  endosperm  and  it  also  functions  as  an 
absorbing  organ  of  the  embryo  (Fig.  86,  D).  Similar  devices 
for  absorbing  the  storage  foods  and  transferring  them  to  the 
embryo  are  seen  in  the  wheat,  onion,  date,  and  cocoanut.  The 
early  growth  of  the  embryo  goes  on  as  in  the  preceding  examples. 
The  root  first  emerges  and  anchors  the  grain  in  the  soil.  The 
plumule  of  closely  rolled  leaves  forms  a  sharp  bodkin  that  reaches 
up  through  the  soil  with  ease.  This  modification  of  the  plumule 
would  appear  to  be  of  a  decided  advantage  and  you  will  notice 
that  this  method  of  emerging  from  the  soil  is  followed  by  a  great 
many  plants  with  underground  stems;  see  the  large  leaves  of  the 
skunk  cabbage,  may  apple,  Solomon's  seal,  etc.  The  rest  of 
the  grain  remains  in  the  ground  as  in  the  case  of  the  pea. 

57.  The  Two  Phases  in  the  Life  of  a  Plant. — As  has  been 
stated  the  plumule  unfolds  in  all  these  cases  like  a  bud  and 
gradually  develops  into  the  mature  plant.  This  may  be  effected 
in  one  year  or  several  years  may  be  required  to  complete  the 
work.  At  some  time  in  this  growth  flowers  will  appear  bearing 
sporophylls  and  spores  and  thus  we  arrive  again  at  the  starting 
point  in  the  life  history  of  a  plant.  There  are  two  distinct  phases 
or  generations  in  the  life  of  a  plant.  The  formation  of  the 
megaspores  and  microspores  marks  the  beginning  of  the  sexual 
generation  or  gametophyte  which  ends  when  the  male  and  female 
gametes  have  been  developed.  The  formation  of  the  gameto- 
spore  marks  the  beginning  of  the  asexual  generation  or  sporo- 
phyte  which  ends  in  the  sporophylls  when  those  cells,  called  the 
spore  mother  cells,  appear  which  produce  the  micro-  and  mega- 
spores.  These  two  generations  are  also  sharply  distinguished 
from  each  other  by  the  number  of  chromosomes,  page  56,  which 
their  cells  contain.  The  cells  of  the  gametophyte  generation 
contain  only  one-half  the  number  of  chromosomes  found  in  the 
cells  of  the  sporophyte.  generation.  If  the  nuclei  of  male  and 
female  gametes  contain  twelve  chromosomes  (the  number  varies 
in  different  plants)  the  gametospore  which  marks  the  beginning 
of  the  asexual  generation,  will  contain  twenty-four  chromosomes. 
This  number  will  be  found  in  all  the  cells  of  the  embryo  and 
continue  to  characterize  the  subsequent  development  of  the  sporo- 


NATURE   OF  PLANTS  137 

phyte.  When,  however,  the  spore  mother  cells  appear  in  the 
anthers  and  ovules  it  is  to  be  noted  that  these  cells  in  dividing 
to  form  the  micro-  and  mega-spores  reduce  the  number  of 
chromosomes  by  one-half  so  that  the  nuclei  of  these  spores  and  of 
all  cells  derived  from  them  contain  but  twelve  chromosomes. 
For  this  reason  the  division  of  the  spore  mother  cell  is  referred 
to  as  the  reducing  division.  The  significance  of  this  difference 
in  the  number  of  chromosomes  is  not  known  but  it  furnishes  a 
basis  for  the  sharp  separation  of  the  two  generations. 

A  sexual  and  an  asexual  phase  or  generation  characterize 
the  life  history  of  nearly  all  the  higher  plants  and  the  succes- 
sion of  these  two  phases  is  called  the  alternation  of  generation. 
In  the  case  of  the  bean  we  have  as  the  most  important  features 
of  the  sporophyte  or  asexual  generation,  (i)  the  gametospore, 
(2)  the  seed,  and  (3)  the  flowering  bean  plant.  The  micro- 
scopic plants  that  comprise  the  gametophyte  or  sexual  genera- 
tion are  characterized  by  (i)  the  micro-  and  mega-spores  which 
develop  respectively  into  (2)  a  tubular  growth  of  three  cells 
and  (3)  a  sac-like  growth  of  seven  cells,  each  of  which  contains 
one  or  more  (4)  sexual  cells  or  gametes.  These  two  phases  are 
called  respectively  the  sporophyte  or  asexual  generation,  and  the 
gametophyte  or  sexual  generation,  since  in  the  first  phase  mother 
cells  are  developed  which  form  spores,  i.  e.,  this  is  the  spore 
forming  or  asexual  generation  and  in  the  second  phase  gametes 
or  sexual  cells  are  produced. 

58.  Significance  of  Fertilization. — Let  us  now  stop  to  con- 
sider the  meaning  of  the  complicated  process  that  we  have  termed 
fertilization.  No  satisfying  explanation  has  been  offered.  Some 
see  in  these  changes  only  a  process  of  nutrition  and  chemical 
stimulation.  The  gametes  are  lacking  in  certain  materials  that 
are  essential  to  their  further  growth.  According  to  this  view 
the  male  gamete  is  the  complement  of. the  female  and  by  the 
union  of  the  two  all  the  substances  are  supplied  that  are  neces- 
sary for  growth. 

It  has  also  been  suggested  that  the  growth  culminating  in 
fertilization  is  attended  with  the  removal  of  impurities  from  the 
sexual  cells.  Every  cell  has  its  growth,  maturity,  and  senescence 
10 


138  PRINCIPLES   OF   HEREDITY 

when  it  loses  its  power  for  further  growth  and  division.  The 
reduction  division  and  the  subsequent  formation  of  the  gametes 
results  in  the  removal  of  substances  that  prevent  their  continued 
activity  and  as  a  consequence  they  are  restored  to  a  youthful 
condition  which  appears  in  the  growth  of  the  gametospore. 

Much  attention  has  been  directed  in  recent  years  to  the  process 
of  fertilization  as  a  method  of  controlling  the  character  of  off- 
spring. You  have  already  noted,  page  56,  that  granular  bodies 
or  chromosomes  are  constant  features  of  every  nucleus.  These 
complex  bodies  determine  the  type  of  plant  that  shall  be  de- 
veloped. In  other  words  the  chromosomes  contain  the  hereditary 
substances,  termed  factors,  that  cause  each  plant  to  develop  in  a 
certain  definite  way  or  to  follow  a  certain  pattern  in  its  growth. 
The  chromosomes  derived  from  the  parent  plants  cause  the  off- 
spring to  resemble  them  in  growth.  The  micro-  and  mega-spores 
are  formed  in  the  sporophylls,  and  these  spores  must  therefore 
contain  the  same  kind  of  chromosomes  or  factors  as  the  plants 
bearing  the  sporophylls,  i.  e.,  the  parent  plant,  since  they  have 
been  derived  directly  from  the  parent  plant  by  cell  division. 
Therefore  gametes  derived  from  the  germinating  spores  must 
also  contain  the  same  factors  as  the  parent  plants.  In  fertiliza- 
tion we  have  the  fusion  of  the  gametes.  This  means  that  the 
hereditary  substances  in  the  gametospore  will  be  of  a  dual 
character  since  it  has  been  derived  from  the  male  and  female 
gametes  (Fig.  80).  Consequently  there  is  now  lodged  in  one 
body,  the  gametospore,  all  the  possibilities  of  growth  that  each 
gamete  inherited  from  the  parent  plant. 

Let  us  now  examine  some  of  the  evidence  indicating  that  the 
hereditary  substances  or  factors  are  distributed  in  a  very  definite 
way  from  parent  to  offspring  and  that  these  factors  are  associ- 
ated with  the  chromosomes.  In  the  common  garden  plant,  four- 
o'clock,  we  find  some  plants  bearing  only  white  flowers,  others 
only  red  flowers.  Here  we  have  evidence  of  two  factors ;  the  one 
causing  the  white  color,  the  other  causing  the  red  color.  If  these 
two  plants  be  crossed  the  resulting  offspring  is  pink  or  inter- 
mediate between  the  two  parents.  This .  offspring  is  a  hybrid, 
meaning  thereby  an  individual  derived  from  parents  that  differ 


NATURE  OF   PLANTS 


139 


from  one  another  in  one  or  more  factors.  If  now  one  of  the 
hybrid  four-o'clocks  is  crossed  with  the  pure  white,  one  half  of 
the  resulting  offspring  will  be  pure  white  and  one  half  pink 
like  the  original  hybrid.  The  reason  for  this  variation  in  the 
offspring  will  be  apparent  by  studying  the  diagram,  Fig.  86,4. 
Here  we  see  that  each  parent  (RR  and  WW}  produced  gametes 
having  the  same  color  factor  as  itself  and  when  two  gametes 

WWe 


TVnfr  Hybrid -> (R    W\ 

W 


FIG.  86-4.  Diagram  showing  distribution  of  the  factors  for  red  (R)  and 
for  white  (W)  in  the  cross  of  the  red  and  white  four-o'clock;  and  the  subsequent 
cross  of  the  pink  hybrid  with  the  white  form. 

derived  from  these  parents  unite  the  resulting  hybrid  (RW) 
must  contain  the  color  factors  of  both  parents.  Note,  however, 
that  the  hybrid  produces  gametes  like  those  of  the  parents,  i.  e., 
gametes  with  the  red  factor  or  with  the  white  factor.  This  is 
due  to  the  fact  that  in  the  formation  of  the  gametes  there  is  a 
separating  out  of  the  factors  so  that  the  gametes  are  never 
hybrid.  They  only  contain  the  factors  of  one  or  the  other  of 
the  original  parents.  The  lower  part  of  the  diagram  shows  how 


140  TRANSMISSION  OF  FACTORS 

the  results  of  crossing  the  pink  hybrid  (RW)  with  the  pure 
white  (PFTF)  come  about.  The  white  form  can  only  produce 
gametes  with  the  white  factor  but  we  have  seen  that  the  hybrid 
produces  gametes,  one  half  of  which  contain  the  white  factor  and 
one  half  the  red  factor.  Therefore  in  the  cross  between  the  pink 
hybrid  and  the  pure  white  there  are  only  two  possible  combina- 
tions of  gametes.  A  gamete  from  the  hybrid  bearing  the  white 
factor  may  fuse  with  a  gamete  from  the  white  four  o'clock,  giving 
a  pure  white  offspring  or  a  gamete  from  the  hybrid  bearing  the 
red  factor  may  fuse  with  a  gamete  from  the  white  four  o'clock, 
giving  a  pink  hybrid. 

It  is  now  known  that  the  hereditary  substances  or  factors  are 
associated  with  the  chromosomes  and  that  they  are  separated 
in  a  very  definite  way  during  the  cell  divisions  that  result  in  the 
formation  of  the  gametes.  These  chromosomes  are  associated 
in  pairs  and  in  the  division  of  the  cell  one  member  of  a  pair  goes 
to  one  daughter  cell  and  the  other  member  of  this  pair  goes  to 
the  other  daughter  cell.  If  now  the  factor  for  red  is  in  one  member 
of  a  pair  of  chromosomes  and  the  factor  for  white  is  in  the  other 
member  of  this  pair,  it  must  follow  in  the  above  cell  division  that 
one  daughter  cell  will  contain  the  red  factor  and  the  other 
daughter  cell  will  contain  the  white  factor.  Therefore  the 
gametes  derived  from  these  two  daughter  cells  must  contain 
respectively  the  red  factor  and  the  white  factor.  This  distri- 
bution of  the  factors  will  be  more  manifest  by  examining  the 
accompanying  diagram,  Fig.  86  B.  In  figure  I  is  shown  a 
nucleus  with  three  pairs  of  chromosomes,  one  pair  very  small, 
another  somewhat  larger  and  a  third  pair  in  which  the  factors  for 
red  and  white  are  indicated  by  the  letters  R  and  W.  Of  course 
it  is  not  to  be  supposed  that  any  factor  is  visible  to  the  eye.  In 
the  second  figure  the  two  members  of  each  pair  of  chromosomes 
is  seen  drawing  together  preparatory  to  cell  division.  In  the 
third  figure  cell  division  is  under  way  and  the  chromosomes  still 
in  pairs  have  been  drawn  to  the  center  of  the  spindle,  and  in  the 
fourth  figure  we  see  a  member  of  each  pair  being  pulled  to  the 
opposite  pole  of  the  spindle,  where  they  will  be  organized  into 
two  daughter  nuclei.  The  later  appearance  of  these  two  daughter 


NATURE  OF   PLANTS 


141 


nuclei  is  shown  at  the  right  and  left  of  figure  four.  In  this  way 
the  red  factor  comes  to  be  lodged  in  one  of  the  daughter  cells 
and  the  white  factor  in  the  other  daughter  cell.  This  separation 
of  the  factors  that  appear  in  the  two  members  of  a  pair  of  chromo- 
somes is  termed  the  segregation  of  the  factors.  It  is  one  of  the 
most  important  features  in  the  cell  divisions  that  result  in  the 


w 

FIG.  865.     Diagram  of  a  nucleus  showing  mode  of  division  in  gamete 
formation.     See  text. 

formation  of  the  gametes.  The  members  of  a  pair  of  chromo- 
somes are  termed  homologous  chromosomes  and  the  contrasting 
factors  which  they  bear,  in  this  case  red  and  white,  are  named 
allelomorphs.  We  see  that  it  is  the  behavior  of  the  homologous 
chromosomes  and  their  allelomorphic  factors  that  determine  the 
nature  of  the  gametes. 

The  fundamental  features  to  note  in  the  above  discussion  is 
first  that  the  factors  remain  distinct  and  show  no  indication  of 
fusing  with  one  another.  The  red  and  white  factors  appear 
unchanged  in  the  gametes  no  matter  how  many  crossings  have 
been  effected.  The  second  feature  is  that  these  factors,  though 
associated  in  pairs  in  the  homologous  chromosomes,  undergo  at  a 


142  PLANT    BREEDING 

certain  stage  of  cell  division  a  separation  or  segregation.  These 
two  principles  of  non-mixing  and  of  segregation  of  factors  were 
discovered  by  Mendel  and  they  are  among  the  most  important 
and  fundamental  facts  in  biological  science.  As  a  single  illustra- 
tion of  this  note  that  the  success  in  breeding  is  solely  based  upon 
Mendel's  principles  of  heredity.  Desirable  characters  of  one 
individual  may  by  crossing  be  associated  with  the  desirable 
characters  of  another  individual  and  in  the  same  way  undesirable 
traits  may  be  eliminated.  These  new  combinations  of  char- 
acters may  be  effected  in  some  cases  with  such  certainty  that 
horticulturists  have  advertised  new  forms,  describing  their 
characters  and  qualities  before  they  have  been  bred.  Late  fruit- 
ing grains  have  been  made  to  mature  earlier,  species  susceptible 
to  cold  have  become  hardy,  as  in  the  case  of  wheat,  which  may 
now  be  cultivated  in  Canada  many  miles  north  of  its  former  range, 
and  non-resistant  forms  have  become  immune  to  disease.  The 
annual  loss  from  a  single  plant  disease,  one  of  the  rusts,  is  esti- 
mated at  500  million  dollars.  In  the  same  way  the  percentage 
composition  of  the  reserve  food  in  grains  has  been  changed  and 
the  resulting  yield  greatly  increased. 

Another  important  feature  in  the  transmission  of  the  hereditary 
factors  is  illustrated  in  the  crossing  of  a  blue-flowered  tall  sweet 
pea  with  a  red-flowered  dwarf  sweet  pea.  In  this  case  we  have 
two  pairs  of  allelomorphic  factors  instead  of  one  as  in  the  four- 
o'clock.  One  member  of  a  pair  of  chromosomes  contains  the 
factor  for  blue  and  the  other  homologous  member  of  this  pair 
contains  the  factor  for  red.  In  another  pair  of  chromosomes 
there  are  lodged  in  the  same  way  the  factors  for  tallness  and 
dwarf  ness.  When  these  two  plants  are  crossed  we  find  that  the 
factors  segregate  as  in  the  first  example  but  the  hybrids  resulting 
from  the  cross  are  never  intermediate  between  the  two  parents. 
It  will  be  noticed  in  a  hybrid  from  this  cross  containing  the  factors 
for  tallness  and  dwarfness  that  the  plant  is  always  tall  like  the 
tall  parent;  so  in  a  hybrid  where  blue  and  red  are  associated  the 
flower  is  always  blue.  It  would  look  as  though  the  factors  for 
dwarfness  and  redness  in  these  cases  had  been  obliterated. 
Such,  however,  is  not  the  case.  They  have  been  inhibited  in 


NATURE  OF  PLANTS  143 

their  action  in  some  way  by  the  opposite  allelomorphic  factor 
and  reappear  in  the  offspring  when  hybrid  tall  blue  sweet  peas 
are  crossed  among  themselves.  In  cases  of  this  nature  where 
certain  factors  are  suppressed  and  others  are  active,  the  inactive 
factors  are  termed  recessive  and  the  active  ones  are  termed 
dominant.  Tallness  and  blue  are  dominant  factors,  dwarfness 
and  red  are  recessive  factors. 

In  the  above  examples  we  see  that  the  hereditary  factors  are 
segregated  and  distributed  to  the  gametes  in  a  very  definite 
way.  There  is  one  important  exception  to  this  rule.  A  large 
number  of  instances  is  known  where  one  factor  is  usually  or 
always  associated  with  another  factor.  That  is,  these  two 
factors  are  not  separated  from  one  another  in  the  formation  of 
the  gamete  and  consequently  both  are  transmitted  to  the 
gamete.  Factors  that  behave  in  this  way  are  said  to  be  linked 
and  linkage  is  to  be  contrasted  with  the  segregation  mentioned 
above.  One  of  the  most  striking  illustrations  of  linkage  is  seen 
in  color  blindness.  The  factor  for  this  characteristic  is  always 
linked  with  the  factor  for  sex.  Therefore  this  factor  is  distributed 
differently  to  the  two  sexes.  The  offspring  of  a  color-blind  male 
and  a  normal  female  would  not  show  this  characteristic  at  all 
because  the  sons  are  absolutely  normal  and  the  daughters,  though 
having  the  factor  for  color  blindness,  do  not  show  it,  since  this 
factor  is  recessive  to  the  normal  sight  factor.  The  daughters, 
therefore,  can  alone  transmit  this  factor,  and  one  half  of  their  sons 
by  a  normal  male  will  be  color  blind. 

In  the  above  examples  very  simple  cases  have  been  taken  in 
order  to  bring  out  more  clearly  the  principles  involved.  We 
have  spoken  of  only  one  factor  being  lodged  in  a  chromosome.  It 
is  now  known  that  many  factors  may  be  associated  with  each 
chromosome  and  the  evidence  at  present  makes  very  probable 
the  belief  that  these  factors  are  arranged  in  a  line  in  the  chromo- 
some and  are  separated  from  each  other  by  definite  distances. 
The  factors  have  also  been  spoken  of  as  though  they  were  the 
sole  cause  of  the  characters.  The  factor  for  redness  is  merely 
the  deciding  cause  of  this  color.  A  great  many  factors  are 
really  necessary  to  cause  the  production  of  a  given  character, 


144  ORIGIN   OF  PLANT  VARIATIONS 

some  being  absolutely  necessary,  others  having  only  slight  effect 
upon  it.  The  character  is  merely  the  end  product  of  a  series  of 
reactions.  Therefore  you  are  not  to  think  of  these  characters 
as  definite  chemical  substances  that  are  all  lodged  in  the  indi- 
vidual at  the  start  of  its  life  and  that  maintain  their  identity 
unchanged  throughout  its  development.  At  the  start  of  an 
organism  a  definite  number  of  factors  are  involved — just  enough 
to  give  direction  to  its  growth  and  to  cause  it  to  follow  a 
pattern  of  development  that  is  peculiar  to  its  kind.  As  growth 
goes  on  owing  to  the  interaction  of  these  factors  and  also  owing  to 
the  action  of  the  environment  characters  are  constantly  created 
and  here  and  there  in  the  organism  characters  appear  that  give 
us  ocular  evidence  of  the  presence  of  the  factors  which  are  the 
deciding  cause  of  a  color,  of  a  form,  of  a  size,  etc. 

It  will  be  seen  from  the  above  discussion  that  no  cross  can 
result  in  a  new  creation.  All  the  work  in  breeding  shows  that 
the  crosses  result  merely  in  new  combinations  of  the  old  factors 
that  already  existed  in  the  parent  stock.  How  then  does  vari- 
ation come  about  and  new  characters  arise?  de  Vries  has 
shown  that  one  or  more  new  characters  may  suddenly  appear  in 
the  offspring  through  the  influence  of  unknown  causes.  These 
sudden  changes  are  termed  mutations.  It  appears  reasonable 
that  the  gradual  accumulations  of  these  mutations  going  on 
over  a  long  period  of  time  may  have  been  the  principal  cause  of 
the  enormous  number  of  variations  that  are  seen  in  plant  and 
animal  life. 

What  then  is  the  purpose  of  fertilization?  We  see  that  it  is  a 
process  for  effecting  crossing  and  so  bringing  about  a  new  combi- 
nation of  the  characters  and  the  variations  that  appear  in  the 
parent  plants.  Some  of  these  new  combinations  will  be  of  no 
value,  others  will  give  their  possessors  an  advantage  because 
they  adapt  them  to  the  conditions  under  which  they  live. 
These  latter  forms  will  therefore  tend  to  survive  and  to  crowd 
out  the  less  favored.  This  is  natural  selection,  the  environment 
bringing  about  the  survival  of  the  fit.  As  long  as  the  environ- 
ment remains  constant  and  natural  selection  has  picked  out  forms 


NATURE  OF   PLANTS  145 

that  are  adapted  to  it,  there  is  no  need  of  further  crossing. 
Indeed  we  see  many  plants  that  flourish  without  crossing.  Owing 
to  the  changes,  however,  that  have  occurred  on  the  earth  since 
plants  first  appeared  there  has  been  a  necessity  for  fertilization 
in  order  that  new  combinations  might  arise  and  adapt  the 
offspring  to  the  new  conditions. 


PART  II 
DEVELOPMENT  OF  PLANTS 


CHAPTER  V 

CLASSIFICATION   OF   PLANTS 

59.  The  Method  of  Classifying  Plants. — We  have  now  arrived 
at  an  understanding  of  the  nature  of  the  plant  and  the  work 
which  it  performs.  In  this  study  the  most  complex  forms  have 
been  largely  considered.  There  are,  however,  a  great  variety  of 
very  simple  forms  ranging  from  single-celled  plants  that  often 
form  the  green  coatings  on  trees  up  to  the  complex  types.  How 
have  these  different  forms  come  about  and  what  relation  do  they 
sustain  to  one  another?  We  will  also  be  interested  to  compare 
the  life  histories  of  these  different  forms  and  their  modes  of  life. 
In  approaching  this  subject  it  is  first  necessary  to  gain  a  know- 
ledge of  the  system  employed  in  grouping  or  classifying  plants. 
Obviously  many  plants  are  closely  related  as  the  red.  white, 
scarlet  oaks.  Such  a  group  of  closely  related  plants  is  called  a 
genus  (plural,  genera)  and  the  different  kinds  of  individuals 
which  compose  the  group  are  known  as  species.  The  red  oak  is 
one  of  the  species  of  the  oak  genus.  In  scientific  work  the  Latin 
or  Greek  names  are  used  to  designate  the  genera  and  species. 
In  the  case  of  the  oak  the  genus  is  known  as  Quercus,  an  old 
Latin  name  for  oak,  and  the  red  oak  species  as  rubra,  meaning 
red.  Both  the  name  of  the  species  and  the  genus  are  employed 
in  naming  a  plant  and  consequently  the  scientific  name  of  the 
red  oak  is  Quercus  rubra.  You  will  frequently  see  a  letter  or  one 
or  more  abbreviations  after  the  scientific  name,  indicating  the 
person  or  persons  who  are  responsible  for  giving  the  name  now 
in  use.  Thus  Quercus  rubra  L.  indicates  that  this  name  was 

146 


DEVELOPMENT  OF  PLANTS  147 

given  by  Linnaeus.  So  in  other  cases  we  can  say  white  oak 
or  Quercus  alba,  scarlet  oak  or  Quercus  coccinea,  etc.  The  oak 
has  several  characters  in  common  with  the  chestnut  and  beech- 
nut genera,  as  for  instance  the  fruit  in  each  case  is  associated 
with  outgrowths  that  appear  as  the  burr  in  the  chestnut  and 
beech  and  as  a  cup  in  the  oak.  On  account  of  these  and  other 
common  characteristics  these  genera  are  supposed  to  be  related 
and  are  therefore  grouped  together  in  one  family.  A  family 
is  composed  of  allied  or  related  genera.  The  name  of  the  family 
is  derived  from  some  characteristic  genus  in  the  group.  In 
this  case  it  happens  to  be  the  beech  or  Fagus.  To  distinguish 
a  family  from  a  genus  the  termination  aceae  is  added  to  the  base 
of  the  generic  word,  in  this  example  making  the  family  name 
Fagaceae.  So  we  have  the  three  allied  genera,  oak,  beech,  and 
chestnut  forming  the  beech  family  or  Fagaceae.  In  the  same 
way  it  will  be  found  that  families  are  related  and  joined  together 
into  a  still  larger  group  known  as  the  order.  The  flowers  of 
the  birch,  alder,  water  beech,  and  hazel  genera  are  very  similar 
and  consequently  form  a  family  known  as  the  birch  family  or 
Betulaceae,  Betula  being  the  name  of  the  birch  genus.  It  will 
also  be  noticed  that  there  is  a  similarity  between  the  flowers  of 
the  Betulaceae  and  Fagaceae.  This  relationship  is  expressed  by 
placing  them  in  the  same  order  known  as  the  beech  or  Fagales, 
the  termination  ales  being  employed  to  distinguish  the  order  just 
as  aceae  characterized  the  family.  So  also  orders  are  related 
and  grouped  into  classes.  For  example,  the  Fagales  and  many 
other  orders  are  characterized  by  having  seeds  with  two  cotyle- 
dons. Several  other  orders  have  seeds  with  but  one  cotyledon. 
This  relationship  is  expressed  by  grouping  the  orders  into  two 
classes,  Dicotyledones  and  Monocotyledones.  In  both  of  these 
classes  the  seeds  are  inclosed  within  the  pistil,  but  in  the  cone- 
bearing  trees  the  seeds  are  exposed  on  a  flat  scale-like  organ.  All 
seeds  producing  plants  belong  to  one  or  the  other  of  these  two 
groups.  So  we  have  two  subdivisions,  the  Angiospermae,  mean- 
ing seeds  inclosed,  and  the  Gymnospermae,  meaning  naked  seeds. 
If  now  we  stop  to  consider  still  larger  groups  than  subdivisions, 
it  will  be  noted  that  many  plants  do  not  produce  seeds,  as  in  the 


148  RELATIONSHIP   OF   PLANTS 

case  of  the  ferns  and  mosses,  etc.  So  a  final  grouping  of  plants 
into  divisions  or  subkingdoms  may  be  made.  All  seed-bearing 
plants  form  the  division  Spermatophyta,  meaning  seed  plants; 
the  ferns  comprise  the  division  Pteridophyta,  meaning  fern  plants, 
etc.  The  termination  of  phyta,  from  phyton  a  plant,  distin- 
guishes the  division  from  the  other  groups.  To  repeat,  the 
division  is  composed  of  subdivisions  which  in  turn  are  made 
up  of  classes.  Frequently  the  division  contains  only  classes. 
Classes  consist  of  related  orders.  The  orders  are  divided  into 
families  which  include  allied  genera  and  these  latter  groups  com- 
prise closely  related  individuals  or  species.  The  vegetation  of  the 
earth  is  separable  into  four  widely  differing  divisions:  I.,  The 
Thallophyta,  including  among  others  the  fungi  and  algae;  II., 
The  Bryophyta  or  moss  plants;  III.,  Pteridophyta  or  fern  plants; 
IV.,  The  Spermatophyta  or  seed  plants.  The  character  and 
relationship  of  these  groups  will  be  considered  in  the  following 
pages. 


CHAPTER  VI 
DIVISION  I.     THALLOPHYTA 


60.  Classification  of  the  Thallophyta. — The  Thallophyta  com- 
prise a  multitude  of  plants  that  include  the  most  primitive  and 
simple  forms  of  vegetation  upon  the  earth.     They  range  in  size 
from  single  microscopic  cells  to  forms  that  are  comparable  in 
bulk  to  some  of  our  shrubs  and  trees.     They  are  all  of  simple 
structure  and  do  not  possess  roots,  stems  and  leaves  in  the  sense 
of  the  seed  plants.     Such  a  type  of  plant  body  is  called  a  thallus 
and  it  assumes  a  variety  of  forms.     The  absorption  and  manu- 
facture of  foods  is  carried  on,  as  a  rule,  by  any  and  all  of  the 
cells  and  this  is  equally  true  of  the  reproductive  process.     This 
Division  is  not  a  natural  one  in  that  some  of  the  subdivisions  are 
not  related  so  far  as  we  know.     Several  of  the  groups  represent 
distinct   lines   of  development   and   they   are   placed   together 
simply  because  they  are  all  simple  forms  of  plant  life.     So  it  is 
impossible  to  give  in  a  short  space  a  general  idea  of  the  nature 
and  character  of  this  division  which  includes  many  groups  of 
widely  different  forms.     For  the  purpose  of  study  the  Thallo- 
phyta may  be  divided  into  five  subdivisions: — (i)  Myxomycetes 
or  Slime  Moulds;  (2)  Schizophyta  or  Bacteria  and  Blue  green 
Algae;   (3)   Diatomeae  or  Diatoms;   (4)   Euphyceae  or  Algae; 
(5)  Eumycetes  or  Fungi. 

Subdivision  i.     Myxomycetes  or  Slime  Moulds 

61.  The  Life  History  of  a  Slime  Mould. — In  one  stage  of  their 
life  the  slime  moulds  have  a  motility  and  mode  of  feeding  sug- 
gestive of  some  of  the  lower  animals,  while  on  the  other  hand 
the  final  stage  of  their  existence  is  more  suggestive  of  the  fungi. 
For  this  reason  it  has  often  been  suggested  that  they  are  inter- 
mediate between  plant  and  animal  life.     These  plants  are  widely 
distributed  over  the  earth  and  may  frequently  be  found  on  de- 
caying logs  and  rotting  twigs  and  leaves  in  forests.     A  common 

I49l 


150 


LIFE  OF  A  SLIME   MOULD 


form,  resembling  a  miniature,  brownish  puffball,  is  often  seen  on 
stumps  and  fallen  logs  (Fig.  87).  Other  kinds  are  illustrated  in 
Fig.  88.  They  range  in  size  from  scarcely  a  pin  head  to  nearly 
a  foot  in  diameter  and  from  spherical  to  cylindrical  and  cake-like 
masses.  Not  infrequently  they  are  of  great  beauty  owing  to  their 
coloration  and  lace-like  structures.  These  small  sacs  or  spor- 
angia (sing,  sporangium)  as  they  are  commonly  called,  repre- 
sent but  one  stage  in  the  life  of  the  slime  moulds.  If  we  begin 
with  an  examination  of  the  dust  that  floats  away  from  these 


FIG.  87.  FIG.  88. 

FIG.  87.  The  sporangial  stage  of  two  common  slime  moulds:  A,  Ly co- 
gala.  B,  Arcyria.  C,  sporangia  rupturing,  hair-like  structures  (the  capil- 
litium)  and  spores  protruding.  D,  sporangia  emptied. 

FIG.  88.  Open  types  of  sporangia:  A,  Stemonitis,  at  left  a  single  spo- 
rangium enlarged,  showing  net-like  structure  formed  by  capillitium  radiating 
from  the  central  stalk  of  the  sporangium.  B,  Cribaria. 

sacs  every  time  they  are  tapped,  the  life  history  will  be  found  to 
be  about  as  follows:  Under  the  microscope  the  particles  of  dust 
are  seen  to  be  minute  cells  or  spores  (Fig.  89,  ^4).  You  may  think 
of  a  cell  as  a  cube,  a  sphere  or  as  assuming  almost  any  form, 


DEVELOPMENT   OF   PLANTS  151 

frequently  greatly  elongated.  It  is  bounded  by  a  wall  and  within 
is  the  living  substance  or  protoplasm.  The  more  important  parts 
of  the  living  substance  are  a  viscid  rather  watery  material  termed 
the  cytoplasm  and  a  minute  denser  body,  the  nucleus,  which  is 
immersed  in  the  cytoplasm.  A  spore  is  a  special  kind  of  cell 


FIG.  89.  FIG.  90. 

FIG.  89.  Mobile  phase  in  life  of  slime  mould:  A,  group  of  spores.  B, 
germination  of  a  spore.  C,  two  forms  of  zoospores  greatly  enlarged.  The 
right-hand  one  mobile  owing  to  a  streaming  movement  of  its  protoplasm 
and  the  left-hand  one  moves  about  owing  to  the  rapid  movements  of  the 
cilium,  c.  D,  association  of  the  zoospores  preliminary  to  the  formation  of 
a  plasmodium.  E,  plasmodium.  The  arrow  indicates  the  direction  of  the 
streaming  movement. 

FIG.  90.  Character  of  the  capillitium:  A,  in  Arcyria.  B,  in  Trichia,  por- 
tion of  thread  enlarged  on  the  left  hand. 

that  is  capable  of  germinating  under  favorable  conditions  and 
producing  a  plant.  In  this  case  it  is  advisable  when  germinating 
the  spores  to  place  them  in  a  nutrient  solution  made  by  boiling 
in  water  bits  of  wood  upon  which  the  sporangia  were  growing. 
Germination  will  begin  in  about  one  day.  As  the  spore  wall 
ruptures  the  contents  escapes  as  a  naked  bit  of  protoplasm 
(Fig.  89,  B)  which  possesses  a  singular  power  of  motion.  This 
motility  is  due  either  to  the  slow  creeping  or  streaming  motion 


152         MOVEMENTS   OF  THE   SLIME   MOULDS 

of  the  soft  plastic  body  or  more  commonly  the  body  may  possess 
a  delicate  thread  or  cilium  (plu.  cilia)  (Fig.  89,  C).  These 
ciliated  bodies  are  called  zoospores.  The  cilium  is  a  highly 
sensitive  organ  which  by  rapid  and  rhythmical  movements, 
somewhat  like  the  movements  of  the  arms  in  swimming,  beats  the 
water  and  thus  drives  the  body  along.  While  in  this  condition 
the  zoospores  are  rapidly  multiplied  owing  to  the  repeated  split- 
ting of  the  little  bodies  into  two  similar  parts.  Finally  this 
greatly  increased  number  of  individuals  begins  to  come  together 
in  small  groups  (Fig.  89,  D)  which  in  turn  merge  and  so  form 
large  slimy  masses  termed  plasmodia  (sing,  plasmodium)  which 
are  quite  destitute  of  all  walls  and  consist  of  a  great  number  of 
nuclei  surrounded  by  a  watery  cytoplasm  (Fig.  89,  E).  You  will 
often  find  this  plasmodial  stage  of  the  slime  mould  on  the  under 
side  of  rotting  limbs  or  bits  of  wood  in  the  forest  as  a  sticky  mass 
resembling  in  consistency  the  white  of  an  egg.  The  mucilaginous 
character  of  the  plasmodium  accounts  for  the  name  slime  mould 
popularly  applied  to  these  plants,  also  for  the  term  myxomycetes, 
from  myxos,  slime,  and  myces,  mould  or  fungus. 

The  plasmodium  possesses  a  sensitiveness  or  capability  of 
responding  to  external  stimuli  to  a  degree  that  is  remarkable 
when  we  consider  the  extreme  simplicity  of  these  plants.  For 
example,  it  avoids  too  strong  a  light  and  slowly  moves  toward 
moisture  and  food.  This  accounts  for  your  failure  ordinarily 
to  notice  it  since  it  seeks  the  darkness  and  food  in  decaying 
logs  or  the  under  surface  of  sticks  and  leaves.  The  motion  is 
brought  about  by  little  arm-like  branches  that  flow  from  the 
jelly-like  plasmodium  and  finally  the  entire  plasmodium  will 
slowly  follow  along  these  lines  with  a  complicated  streaming 
movement  (Fig.  89,  E).  If  bits  of  wood  containing  some  of 
the  plasmodium  are  placed  in  a  damp  chamber  in  the  dark  after 
a  time  the  plasmodium  will  be  seen  creeping  up  the  sides  of  the 
moist  dish  in  a  complex  skein-like  mass  or  if  a  glass  slide,  down 
which  a  very  meager  amount  of  water  is  slowly  allowed  to  trickle, 
is  brought  in  contact  with  the  plasmodium  it  may  be  observed 
creeping  up  the  slide  toward  the  source  of  the  water  supply. 
The  slime  moulds  do  not  contain  chlorophyll  and  are  therefore 


DEVELOPMENT   OF   PLANTS  153 

dependent  upon  organic  food.  Plants  that  feed  in  this  way  are 
.called  saprophytes.  Since  the  plasmodium  is  not  surrounded  by 
a  cell  wall  feeding  becomes  a  simple  matter.  The  jelly-like 
mass  engulfs  the  decaying  particles  or  other  foods  and  after 
digesting  the  nourishing  portions  leaves  behind  the  worthless 
parts  as  it  creeps  along.  As  the  plasmodium  approaches  the 
final  stage  of  its  life,  its  nature  appears  to  change  completely, 
for  now  it  avoids  moisture  and  seeks  the  light.  It  creeps  to 
the  surface  of  the  wood  or  leaves  in  which  it  has  been  growing  and 
forms  the  characteristic  bodies  seen  in  Figs.  87,  88.  These 
sporangia  are  formed  by  the  outer  part  of  the  plasmodium 
hardening  into  a  wall,  while  each  of  the  inclosed  nuclei,  which 
have  greatly  increased  in  number  by  division,  becomes  sur- 
rounded by  a  wall,  thus  forming  the  spores.  Usually  a  portion 
of  the  substance  of  the  young  sporangium  is  transformed  into 
simple  or  branching  threads  or  tubes,  collectively  called  the  capil- 
litium  (Fig.  90).  In  several  of  the  genera  the  capillitium  forms 
a  network  within  the  delicate  walls  of  the  sporangia  and  owing 
to  the  early  breaking  down  of  the  wall,  the  feathery  frame  alone 
remains  (Fig.  88) .  These  threads  are  hygroscopic  and  their  con- 
stant motion  assists  in  stirring  up  the  spores  and  exposing  them 
gradually  to  the  wind  as  soon  as  the  wall  of  the  sporangium 
ruptures.  A  somewhat  simpler  type  of  slime  mould  lives  as  a 
parasite  in  the  roots  of  turnip,  cabbage  and  cauliflower,  producing 
a  destructive  Disease  known  as  clubroot. 

There  are  several  groups  of  low  types  of  plant  and  animal  life 
that  are  suggestive  of  relationship  with  the  slime  moulds.  The 
most  important  among  these,  the  myxobacteriales,  have  been 
made  known  by  Thaxter.  They  are  minute  plants,  very  sugges- 
tive of  the  next  group,  the  bacteria,  but  associated  in  definite 
structures  that  resemble  the  sporangia  of  the  slime  moulds  and 
also  of  certain  fungi.  On  the  other  hand,  certain  aquatic  forms, 
as  Protomyxa,  with  a  life  history  very  similar  to  that  of  the 
slime  moulds,  intergrade  almost  perfectly  towards  simple  ani- 
mal types,  as  the  protozoans,  of  which  the  common  amoeba  is 
an  example. 

Thus  we  see  that  the  life  history  of  these  plants  is  a  very  simple 
ii 


154  NATURE   OF   BACTERIA 

one.  By  the  formation  of  sporangia  numerous  dust-like  spores 
are  produced  that  are  capable  of  germinating  and  forming 
zoospores  that  increase  rapidly  by  division.  The  aggregation  of 
the  zoospores  results  in  the  formation  of  the  plasmodium  which 
after  a  time  completes  the  life  history  by  creeping  upon  suitable 
dry  objects  and  forming  sporangia.  The  mingling  of  the  zoo- 
spores  in  the  formation  of  the  plasmodium  is  not  a  sexual  process, 
since  the  nuclei  do  not  fuse,  but  Olive  has  shown  that  the  spores 
are  formed  in  the  same  manner  as  in  plants  characterized  by 
sexual  reproduction  and  that  therefore  there  must  be  a  sexual 
fusion  of  the  nuclei  at  some  time  in  the  life  history  of  the  plas- 
modium. The  structure  and  life  history  of  these  plants  is  so 
simple  that  it  is  possible  that  they  may  have  been  derived  from 
forms  allied  to  the  first  forms  of  life  upon  the  earth.  The 
common  occurrence  among  simple  animals  and  plants  of  motile 
gastss  or  zoospores  in  their  life  history  certainly  suggests  that 
possibly  such  forms  may  represent  the  first  appearance  of  living 
matter. 

Subdivision  2.     Schizophyta  or  Bacteria  and  Blue  Green  Algae 

As  in  the  preceding  group  these  forms  are  extremely  simple 
and  possess  some  characters  suggestive  of  the  lower  forms  of 
animal  life.  They  have  received  the  name  of  Schizophyta,  mean- 
ing splitting  plants,  owing  to  their  common  method  of  repro- 
duction by  division  of  the  cell  into  two  equal  parts.  There  are 
two  important  classes  of  Schizophyta:  A,  Bacteria;  B,  The  Blue 
Green  Algae. 

CLASS  A.     BACTERIA 

62.  The  Structure  and  Nature  of  Bacteria. — Bacteria  are  com- 
monly known  by  such  vague  terms  as  microbes  and  germs.  They 
are,  however,  unicellular  plants  that  are  of  almost  universal  dis- 
tribution, though  more  abundant  about  dwellings  and  less  com- 
mon in  cold  countries,  at  high  altitudes  and  on  the  sea.  They 
include  the  smallest  and  simplest  forms  of  plants,  ranging  from 
scarcely  1/50,000  in.  to  1/10,000  in.  in  diameter  (Fig.  91).  Such 
forms  would  have  many  times  more  room  in  a  drop  of  water 
than  a  whale  would  find  in  New  York  harbor.  So  minute  and 


DEVELOPMENT   OF   PLANTS 


155 


simple  in  structure  are  the  bacteria  that  the  real  nature  of  the 
plant  body  is  somewhat  a  matter  of  dispute.  The  plants  are 
unicellular  and  surrounded  by  a  delicate  thin  wall  which  in- 
closes a  colorless  and  slightly  granular  protoplasm  (Fig.  91, 
A,  i).  There  is  no  nucleus  comparable  to  that  of  the  higher 


FIG.  91.  Forms  of  Bacteria:  A,  Bacillus  subtilis,  a  form  common  in  hay 
infusions.  I,  motile  state;  2,  cells  with  spores;  3,  slimy  mass  of  bacteria, 
the  zooglea  condition,  that  appears  on  the  surface  of  infusions,  cooked  veg- 
etables, etc.  B,  Spirillum.  C,  a  coccus  form  that  appears  in  pus.  D,  mobile 
and  spore  stage  of  lock-jaw  bacillus. 

plants,  although  indications  of  it  are  seen  in  a  few  scattered  chro- 
matin  grains.  The  cell  content  is  very  simple  and  totally  lacking 
in  plastids  and  other  differentiations  with  which  you  are  familiar. 
The  slimy  appearance  of  bacteria  noticeable  where  they  grow 
together  in  colonies  is  due  to  the  mucilaginous  excretion  from 
their  bodies  which  is  often  brightly  colored.  Bacteria  range 
from  globular  to  rod-like  and  curved  forms  (Fig.  91).  Many 
are  motile  by  means  of  cilia,  as  in  the  case  of  the  zoospores  of 
the  Myxomycetes,  which  project  singly  or  in  tufts  from  the  ends 
of  the  cells  or  in  varying  numbers  from  all  sides. 

(a)  Reproduction  of  Bacteria. — This  is  a  simpler  process  than 
in  the  slime  moulds  and  much  more  rapidly  effected  when  the 
conditions  are  favorable  for  growth.  This  process  consists  of 


156  EXCLUSION   OF   BACTERIA 

forming  a  partition  through  the  middle  of  a  cell  when  the  two 
daughter  cells  thus  formed  separate  at  once  or  they  may  remain 
attached,  forming  ultimately  a  chain  of  cells,  since  these  two  new 
cells  continue  to  grow  and  repeat  the  dividing  process.  This 
method  of  multiplication  goes  on  with  great  rapidity,  frequently 
within  a  half  hour,  so  that  many  millions  of  plants  may  be  formed 
in  a  day  from  a  single  bacterium.  This  accounts  for  the  rapidity 
with  which  solutions,  as  beef  tea,  in  which  bacteria  thrive, 
become  turbid  and  also  for  the  gelatinous  masses  and  wrinkled 
scum  that  appears  upon  cooked  foods  and  soups,  if  allowed  to 
stand  undisturbed  for  a  day  or  more  (Fig.  91,  A,  3).  Conn 
estimates  that  a  single  bacterium,  if  allowed  to  develop  under 
perfect  conditions,  would  produce  in  three  days  a  mass  of  bac- 
teria weighing  4,700  tons.  When  the  conditions  are  unfavorable 
for  growth  many  species  of  bacteria  have  another  form  of  repro- 
duction and  form  small  bodies,  called  spores,  within  the  cell 
(Fig.  91,  A,  D).  These  spores  are  exceedingly  resistant  to  un- 
favorable conditions  but  germinate  readily,  producing  bacteria 
when  conditions  are  again  suitable  for  growth.  Some  spores  will 
stand  boiling  for  more  than  an  hour.  This  peculiarity  of  the 
spore,  while  of  great  advantage  to  the  bacteria,  renders  them  very 
difficult  to  exterminate. 

(&)  Exclusion  of  Bacteria. — The  various  devices  for  preserving 
fruits  and  meats  and  sterilizing  utensils  and  instruments  in 
surgery  is  based  upon  some  method  of  killing  these  organisms 
and  preventing  the  entrance  of  others.  This  is  the  purpose  of 
boiling  and  canning  foods  and  of  salting  or  drying  fruits  and 
meats.  The  boiling,  if  thorough,  effectually  sterilizes,  while  the 
canning  excludes  all  organisms  and  the  material  will  remain  un- 
changed indefinitely.  Smoking  or  drying,  with  addition  of  salt 
in  the  case  of  meat  and  of  sugar  in  fruits,  is  effectual  since 
bacteria  flourish  only  in  the  presence  of  moisture.  This  is  the 
reason  that  seeds  and  other  structures  are  not  speedily  destroyed 
by  bacteria.  Seeds  are  naturally  dry  and  protected  by  coats  that 
tend  to  exclude  the  moisture  for  some  time.  Sugar  is  used  as  a 
preservative  because  bacteria  cannot  endure  over  40  or  50  per 
cent,  of  it.  Salt  for  the  same  reason  is  useful  in  preserving 


DEVELOPMENT   OF   PLANTS  157 

meat  because  it  is  injurious  to  bacterial  life  and  smoking  also 
introduces  volatile  products  that  prevent  their  growth.  So  too 
acids  are  generally  harmful  to  bacteria.  You  have  heard  much 
regarding  the  use  of  salicylic  acid,  benzoate  of  soda,  etc.,  in  pre- 
serving foods  and  certain  beverages.  Whether  these  substances 
are  harmful  to  us  or  not,  certain  it  is  that  they  are  fatal  to  the 
existence  of  bacteria.  So  also  meats,  fruits  and  vegetables  are 
kept  in  a  wholesome  condition  for  considerable  periods  in  ice 
boxes  and  for  months  in  cold  storage  since  the  bacteria  grow 
more  slowly  as  the  temperature  is  lowered  and  their  activity 
ceases  at  the  freezing  point.  It  should  be  remembered,  however, 
that  no  amount  of  cold  will  kill  all  of  the  bacteria  and  that  food 
spoils  very  quickly  when  removed  from  cold  storage. 

(c)  Economic  Importance  of  Bacteria. — Insignificant  and  simple 
as  are  the  forms  and  structures  of  bacteria  it  is  safe  to  say  that 
no  plants  are  so  vitally  related  to  our  welfare.  Numerous  forms 
are  instrumental  in  effecting  changes  in  various  substances  termed 
fermentation;  others  assist  in  the  decay  of  dead  plants  and 
animals;  a  comparatively  small  number  of  species  build  up  or 
construct  substances  that  are  of  vital  importance  to  plant  life; 
finally  many  diseases  of  plants  and  animals  are  due  to  bacteria. 
The  work  performed  in  all  the  above  mentioned  processes,  save 
the  third,  is  of  the  same  nature.  These  plants  have  the  power 
of  breaking  down  into  simpler  compounds  the  various  sub- 
stances with  which  they  may  be  brought  in  contact.  Thus 
vinegar  is  formed  through  the  decomposition  of  certain  alcohols 
into  simpler  substances,  as  acetic  acid  and  a  gas,  carbon  dioxide. 
These  changes  are  effected  by  substances,  termed  ferments  or 
enzymes,  that  are  excreted  by  the  bacteria.  These  enzymes  are 
very  remarkable  compounds  (p.  20) .  They  do  not  enter  into  per- 
manent union  with  any  substance  but  by  their  presence  cause  it  to 
break  down  into  some  of  its  component  parts.  When  this  decom- 
position is  attended  with  the  liberation  of  gas,  as  in  certain  sugar 
solutions,  we  term  the  process  fermentation  but  when  in  an 
exactly  similar  way  bacteria  break  down  the  substances  compos- 
ing the  bodies  of  plants  and  animals  we  term  the  process  decay. 
So  also  in  disease  bacteria  simply  cause  the  decomposition  of 


158  BACTERIA  AND   FERMENTATION 

certain  of  the  compounds  that  make  up  the  body.  In  some  cases 
these  simpler  products  that  result  from  the  activity  of  bacteria 
are  very  poisonous  and  in  disease  it  is  these  substances,  toxins, 
that  often  cause  the  principal  danger.  Let  us  now  consider  the 
more  important  of  these  operations. 

1.  Fermentation. — A  very  large  group  of  bacteria,  in  common 
with  some  fungi,  live  upon  sugars  and  other  carbohydrates, 
causing  a  decomposition  known  as  fermentation.     Vinegar  is 
due  to  the  decomposition  of  the  alcohol  in  cider  and  other  weak 
alcoholic  solution  by  numerous  species  of  bacteria  which  appear 
as  slimy  masses  and  are  popularly  known  as  mother  of  vinegar. 
Milk  sours  and  coagulates  through  the  agency  of  bacteria  which 
reduce  the  sugar  to  lactic  acid,  which  in  turn  coagulates  casein. 
The  beverages  known  as  zoolack,  kumiss,  etc.,  are  milk  prepara- 
tions soured  by  special  kinds  of  bacteria  or  yeast.     The  Metch- 
nikoff  tablets  contain  bacteria  that  are  especially  active  in  this 
way  and  they  are  also  supposed  to  live  in  the  lower  alimentary 
tract  where  they  form  decomposition  products  that  are  injurious 
to  the  harmful  bacteria  that  live  in  this  same  region.     So  also 
butyric  acid,  necessary  in  the  manufacture  of  cheese,  is  produced 
by  bacteria.     It  will  be  seen  from  this  that  the  products  of  de- 
composition are  not  necessarily  harmful.     It  is  interesting  to 
note  that  some  of  the  flavors  of  cheese  and  of  high  grade  butter 
are  due  to  the  products  of  decomposition  and  the  excretions  from 
bacteria.     It  is  evident  that  this  might  be  true  in  the  case  of 
limburger  cheese  and  rancid  butter  but  also  remember  that  species 
of  bacteria  are  cultivated  and  used  to  impart  certain  flavors  to 
cheese  and  also  to  a  less  extent  to  butter.     It  is  altogether 
probable  that  this  will  become  a  common  practice  in  our  dairies. 

2.  Decay. — The  great  majority  of  bacteria  live  as  saprophytes 
upon   dead   plants   and   animals.     They   begin   their   work   of 
decomposition  immediately  upon  the  death  of  the  organism. 
In  the  case  of  plants  they  are  generally  assisted  at  first  by 
various  fungi.     There  are  numerous  classes  in  this  group,  each 
kind  doing  a  special  work  in  bringing  about  the  decay.     The 
first  steps  in  the  decomposition  are  quickly  performed  and  then 
the  simpler  compounds,  though  mostly  still  quite  complex,  are 


DEVELOPMENT   OF   PLANTS  159 

acted  upon  by  an  entirely  different  class  of  bacteria  that  carry 
on  the  work  to  a  further  stage  of  decay.  So  you  can  think  of 
many  different  kinds  of  bacteria  each  taking  up  the  work  where 
it  was  left  by  its  predecessor  and  carrying  it  a  step  farther.  In 
this  way  the  complex  compounds  that  make  up  the  living  organ- 
ism are  broken  down  into  very  simple  ones,  such  as  water  (H2O) 
or  gases  as  carbon  dioxide  (CO2),  ammonia.  (NH3),  methane 
(CH4),  sulphureted  hydrogen  (H2S),  nitrogen  (N),  etc.  It  is 
not  to  be  understood  that  all  of  these  'simpler  substances  arise 
only  in  the  final  stage  of  decay.  Some  of  them  may  arise  very 
early  in  the  process,  as  ammonia  and  carbon  dioxide,  or  they 
may  be  set  free  at  later  stages  in  the  decay.  Some  of  these 
gases  are  the  cause  of  the  foul  odors  associated  with  decay  and  it 
should  be  added  that  certain  of  the  rather  simpler  products  of 
decomposition,  termed  ptomaines,  are  exceedingly  poisonous. 
The  ptomaines  found  in  fish,  cheese,  ice  cream,  are  decomposition 
products  formed  by  the  bacteria  of  decay.  Unfortunately  these 
substances  are  without  taste  or  odor. 

This  process  of  decay  is  of  importance  from  two  standpoints. 
First,  it  prevents  the  accumulation  of  organic  matter  upon  the 
earth.  Were  it  not  for  these  changes  every  plant  and  animal 
would  remain  unchanged  after  death  and  so  prevent  further 
continuation  of  life.  Second,  we  see  that  the  successive  steps 
in  decay  result  in  the  formation  of  simple  compounds  that  are 
either  directly  or  indirectly  utilized  by  plants  in  the  formation 
of  their  tissues.  Thus  H2O  and  CO2  are  used  by  the  plant  in 
forming  sugars  and  starches  which  furnish  the  materials  that 
go  to  make  up  the  cell  walls  and  also  assist  in  building  up  the 
living  substance.  So  also  the  N  and  the  S  that  we  have  seen 
set  free  as  elements  or  simple  compounds  are  utilized  in  the 
formation  of  the  living  matter  of  the  plant,  though  not  directly, 
as  we  will  see  below. 

3.  Construction  or  Synthesis. — In  sharp  contrast  to  the  above 
mentioned  forms  stands  a  rather  small  group  of  bacteria  that' 
possess  the  remarkable  power  of  taking  certain  of  the  simpler 
products  of  decomposition  and  adding  other  elements  to  them. 
These  forms,  therefore,  are  quite  different  in  their  work  from  other 


160  NITRIFYING   BACTERIA 

bacteria.  They  build  up  substances  instead  of  breaking  them 
down.  This  work  of  combining  two  or  more  elements  is  termed 
synthesis  and  we  may  term  these  forms  the  synthetic  bacteria. 
These  bacteria  are  of  great  economic  importance  and  those  kinds 
that  have  the  power  of  combining  nitrogen  into  a  form  in  which 
it  can  be  used  by  plants  are  of  first  importance.  Nitrogen,  as  it 
appears  in  the  process  of  decay  is  in  a  form  that  the  plant  can  uti- 
lize either  not  at  all  or  only  to  a  limited  extent.  There  are  several 
bacteria  that  have  the  power  of  combining  nitrogen  with  other 
elements  and  so  constructing  a  nitrogen  compound,  termed  a 
nitrate,  that  is  the  most  valuable  crude  food  product  of  plants. 
Among  these  forms  may  first  be  mentioned  the  nitrifying  bacteria. 
They  have  the  power  of  adding  another  element  to  nitrogen 
compounds.  They  are  able  to  add  oxygen  (O)  to  ammonia 
(NH3),  that  is  to  oxidize  it,  and  thus  transform  it  into  a  nitrate. 
This  synthesis  is  accomplished  in  a  truly  remarkable  manner; 
no  less  than  two  forms  working  together  are  required  to  accom- 
plish the  work.  The  first  form  adds  two  atoms  of  oxygen  to 
the  NH  compound,  thus  changing  it  to  a  nitrous  or  nitrite  state 
while  the  second  form  adds  to  the  nitrite  an  additional  atom 
of  oxygen,  thus  completing  the  formation  of  the  nitric  or  nitrate 
condition.  The  reactions  caused  by  these  two  forms  may  be 
expressed  as  follows: 
2NH3  +  3O2  =  2H2O  -f-  2HNO2  (nitrous  form  of  nitrogen), 

2HNO2  +  O2  =  2HNO3  (nitric  form  of  nitrogen). 
These  two  organisms  work  together,  each  doing  its  special  work, 
and  they  are  so  efficient  that  a  solution  of  ammonia  poured  upon 
soil  will  show  no  trace  of  the  ammonia  in  the  solution  after  it  has 
percolated  through  the  soil.  Fully  65  per  cent,  of  the  nitri- 
fication is  effected  in  the  upper  twelve  inches  of  the  soil  and  very 
little  below  a  few  feet.  It  is  probable  that  the  Chilian  saltpeter 
beds,  previously  referred  to,  are  the  remains  of  inland  seas 
where  great  growths  of  seaweeds  accumulated.  Owing  to  the 
drying  out  of  these  bodies  of  water  and  the  decay  of  the  vegeta- 
tion ammonia  was  formed  which  became  changed  to  nitrates  as 
outlined  above.  Were  it  not  for  these  organisms  the  ammonia 
compounds  would  quickly  escape  from  the  soil  as  a  gas  and  thus 


DEVELOPMENT   OF   PLANTS  161 

the  principal  source  of  nitrogen  in  nature  for  the  plant  would  be 
lost. 

You  have  noticed  that  free  nitrogen  (N)  may  also  appear  in 
decomposition.  A  few  forms  of  bacteria  have  been  discovered 
that  are  able  to  synthesize,  this  gaseous  element  into  a  compound 
that  is  of  use  to  plants  in  a  manner  quite  as  remarkable  as  in  the 
case  of  the  nitrifying  bacteria.  Because  they  are  able  to  fix  the 
free  gaseous  element  nitrogen  inja  stable  compound  these  bacteria 
are  referred  to  as  the  nitrogen-fixing  bacteria — in  contradis- 
tinction to  the  nitrifying  bacteria  that  only  act  upon  nitrogen 
when  it  is  already  in  combination,  as  in  ammonia.  The  most 
important  of  the  nitrogen-fixing  bacteria  are  those  that  live  in 
the  roots  of  plants  belonging  to  the  bean  family.  They  cause 
minute  gall-like  swellings,  termed  tubercles,  upon  the  roots  of 
beans,  clovers,  alfalfa,  etc.,  and  are  able  to  combine  the  free 
nitrogen  of  the  air  into  a  nitrogenous  compound  suitable  for  the 
nourishment  of  the  plant.  These  bacteria  are  of  inestimable 
value  in  maintaining  the  fertility  of  the  soil  (see  page  65). 
Less  important  economically  than  the  tubercle-forming  bacteria 
but  more  wonderful  biologically  are  those  forms  that  live  in  the 
soil  quite  independent  of  higher  plants  and  effect  a  somewhat 
similar  fixation  of  free  nitrogen.  In  this  work  they  are  associated 
with  two  other  forms  that  take  no  part  in  the  fixation  of  nitrogen 
but  simply  remove  the  oxygen  and  probably  the  organic  matter, 
for  only  in  the  absence  of  these  substances  can  these  remarkable 
bacteria  live.  In  this  manner  they  are  able  to  fix  the  nitrogen 
and  build  up  complex  compounds,  probably  of  a  proteid  nature, 
which  are  subsequently  decomposed  and  eventually  converted 
into  available  nitrates  by  other  bacteria. 

As  a  final  example  of  these  synthesizing  bacteria  mention 
should  be  made  of  the  iron  and  especially  of  the  sulphur  bacteria. 
They  are  able  to  oxidize  simple  compounds  of  iron  and  sulphur 
so  that  deposits  of  these  minerals  may  often  be  seen  as  slimy 
white,  red  or  yellow  masses  in  springs  and  streams.  The  sulphur 
bacteria  are  of  especial  interest  because  they  are  able  to  oxidize 
one  of  the  products  of  decomposition,  H2S,  into  a  sulphate — one 
of  the  valuable  crude  plant  foods.  These  plants  are  practically 


162  SIGNIFICANCE   OF   BACTERIA 

independent  of  organic  matter  for  their  support,  deriving  the 
necessary  energy  for  their  growth  from  the  decomposition  of 
inorganic  matter.  This  also  is  absolutely  the  case  with  the 
nitrifying  bacteria  which  are  unable  to  live  upon  organic  matter 
and  it  should  be  added  that  these  latter  forms  effect  the  same 
decomposition  of  carbon  dioxide  as  noted  in  photosynthesis 
(see  page  12).  This  is  an  important  fact,  for  we  see  that  there 
are  two  methods  for  effecting  tr^  storing  up  of  energy.  Green 
plants  are  the  principal  agents  in  this  work.  They  build  up, 
synthesize,  such  substances  as  sugars  by  utilizing  the  energy  of 
the  sunlight — for  this  reason  the  process  is  termed  photosynthesis. 
Certain  bacteria  also  build  up  compounds  by  utilizing  the  energy 
of  chemical  reactions.  This  process  is  therefore  termed  chemo- 
synthesis. 

We  should  now  stop  for  a  moment  and  consider  what  the  real 
significance  of  these  bacterial  forms  is.  Green  plants  build  up 
complex  substances  from  certain  elements  and  simple  com- 
pounds. In  the  first  steps  of  this  work  they  may  be  assisted  by 
certain  bacteria  and  fungi.  These  complex  compounds  are  now 
reduced  by  other  bacteria  to  their  original  state  so  that  they  may 
be  used  again  by  the  green  plant.  This  in  a  word  is  the  story 
of  the  organic  world.  Certain  substances  in  the  air  and  in  the 
soil  are  going  through  an  endless  rotation  or  cycle  of  chemical 
changes — ever  changing  from  simple  to  complex,  from  a  com- 
plex state  back  to  their  simple  form.  So  then  the  green  plant  is  a 
temporary  carrier  of  certain  elements  that,  owing  to  their  combi- 
nations, represent  a  certain  amount  of  stored  up  energy.  We 
and  the  plant  and  the  bacteria  utilize  these  substances  in  the 
same  way.  We  eat  them,  i.  e.,  decompose  them.  This  is 
practically  the  only  way  that  life  can  be  maintained  and  the  end 
result  of  this  eating,  this  decomposition,  is  that  the  elements 
making  up  the  compounds  are  returned  unchanged  to  the  earth, 
to  go  again  through  the  cycle  of  changes. 

4.  Disease. — In  contrast  to  the  kinds  of  bacteria  mentioned 
above  there  is  another  group  that  live  as  parasites  on  plants 
and  animals  producing  disease  either  by  destroying  the  tissue 
and  sapping  the  vitality  or  by  the  production  of  poisonous 


DEVELOPMENT   OF   PLANTS  163 

compounds,  toxins.  Among  the  more  terrible  of  these  infec- 
tious bacteria  may  be  mentioned  those  producing  consumption. 
These  affect  especially  the  lungs  of  animals  and  cause,  according 
to  data  now  available,  from  150  to  200  thousand  deaths  annually 
in  the  United  States.  It  is  altogether  probable  with  complete 
returns  that  the  death  rate  will  exceed  by  50  thousand  the  higher 
figure  mentioned  above.  Consumption  is  no  longer  considered 
so  fatal  a  disease  as  formerly.  A  person  of  ordinary  constitution, 
if  suitably  nourished,  can  be  cured  by  living  in  the  open  air. 
This  pest  could  probably  be  wiped  out  within  ten  years,  if  the 
habit  of  spitting  could  be  stopped.  The  spores  and  plants  are 
readily  carried  away  in  the  air  from  the  dried  sputum  to  be 
inhaled  again  and  so  spread  the  disease.  The  bacteria  causing 
lockjaw,  or  tetanus,  are  especially  abundant  in  the  soil  in  certain 
localities.  Through  wounds  they  are  carried  into  the  system. 
These  forms,  like  others  previously  noted,  are  peculiar  in  that 
they  can  not  grow  in  the  presence  of  oxygen.  Consequently  the 
dangers  of  infection  are  great  in  deep  wounds.  An  immediate 
cauterization  of  the  entire  surface  of  the  wound  will  kill  the  or- 
ganisms. Other  well-known  bacterial  diseases  are  pneumonia, 
diphtheria,  erysipelas,  Asiatic  cholera,  typhoid,  and  splenic 
fevers  and  grippe.  It  should  be  noted  that  many  diseases, 
as  smallpox,  malaria,  hydrophobia,  yellow  fever  and  probably 
scarlet  fever,  etc.,  are  due  to  a  low  order  of  microscopic  animal 
life.  Disease-producing  bacteria  are  either  localized,  as  in  the 
lungs,  throat,  intestines,  or  are  generally  distributed  throughout 
the  system.  The  symptoms  of  the  disease  are  largely  due  to 
the  products  of  decomposition  caused  by  the  growth  of  the  bac- 
teria or  by  secretions  from  the  bacteria  themselves,  all  of  which 
act  as  poisons  and  are  commonly  referred  to  as  toxins.  In 
other  cases  the  toxins  are  formed  by  the  infected  animal  owing 
to  the  disturbance  of  its  growth  and  nutrition  by  the  bacteria  in 
its  tissues.  In  the  same  way  bacteria  are  the  cause  of  many 
diseases  among  the  plants,  as  in  corn,  melons,  many  fruits,  etc. 
See  Bacteria  in  Relation  to  Plant  Disease,  by  Erwin  F.  Smith. 

These  bacteria  are  constantly  finding  their  way  into  our  bodies 
by  means  of  the  breath  but  so  long  as  the  system  is  in  a  healthy 


1 64  BACTERIA   OF   DISEASE 

condition  there  is  only  slight  danger  of  the  organism  gaining  a 
foothold.  Just  how  the  system  is  able  to  combat  the  growth  of 
these  plants  is  now  the  subject  of  earnest  investigation.  Certain 
diseases,  as  mumps,  measles,  whooping-cough,  smallpox,  etc., 
usually  occur  but  once  and  it  is  probable  that  substances  are 
formed  in  the  body  as  a  result  of  the  presence  of  the  bacteria  or 
animal  organisms  that  prevent  a  second  growth  of  the  germs  and 
the  individual  thus  becomes  immune  to  further  attacks.  The 
formation  of  such  a  substance  is  known  to  occur  in  the  case  of 
diphtheria.  The  bacteria  of  this  disease  occur  in  the  upper  air 
passages  of  the  throat  and  secrete  a  poison  or  toxine  that  affects 
the  entire  body.  Gradually  the  system  forms  a  substance  called 
an  antitoxine  that  stops  the  growth  and  eventually  kills  the  bac- 
teria. The  vigor  of  the  system  and  the  virulence  of  the  bacteria 
decide  whether  the  disease  shall  prove  fatal  before  the  anti- 
toxines  are  formed  in  sufficient  quantity  to  destroy  the  bacteria. 
Note  that  these  disease-producing  bacteria  vary  in  their  power 
to  produce  toxine.  Sometimes,  as  in  epidemics,  they  are  so 
virulent  as  to  attack  with  fatal  results  the  strongest  individuals. 
In  other  cases  the  bacteria  appear  to  be  degenerate  and  only  pro- 
duce mild  symptoms  of  the  disease.  This  is  the  principle  of 
vaccination.  The  little  animals  causing  smallpox  are  cultivated 
under  conditions  that  weaken  them  "and  render  them  less  virile. 
Consequently  when  they  are  introduced  into  the  system  they  pro- 
duce only  mild  symptoms  of  the  disease.  The  history  of  the 
study  of  bacteria  constitutes  one  of  the  most  interesting  and 
fascinating  pages  in  science.  Pasteur  was  the  first  to  demon- 
strate that  fermentations  and  putrefactions,  and  later  that 
certain  contagious  diseases,  were  due  to  bacteria  and  animal 
organisms.  His  work  is  now  commemorated  by  a  tablet  on  the 
rue  Pasteur  in  Paris  with  the  inscription:  Here  stood  Pasteur's 
laboratory.  1857  Fermentations;  1860  Spontaneous  Genera- 
tion; 1865  Diseases  of  Wines  and  Beers;  1881  Virus  and  Vaccini; 
1888  Silk- worm  Distempers;  1864-1888  Hydrophobic  Remedies. 
To-day  every  state  and  government  has  its  corps  of  workers  and 
every  city  its  expert  bacteriologists  who  are  studying  the  nature 
of  bacteria  and  their  relation  to  plants  and  animals. 


DEVELOPMENT   OF   PLANTS  165 

CLASS  B.     BLUE  GREEN  ALGAE  OR  CYANOPHYCEAE 
63.  The  Structure  and  Nature  of  the  Cyanophyceae. — These 
plants  are  very  simple  organisms  that  have  many  features  in 
common  with  the  bacteria.     They  are  unicellular  plants,   al- 
though the  cells  are  more  commonly  joined  into  rows,  forming  a 
thread  or  filament  (Fig.  92,  A,  B).     In  some  cases  these  filaments 
may  sjpe  regarded  as  multicellular  plants  since  they  branch  and 
because  the  various  cells  perform  different  functions.     They  form 
slimy,  blue-green,  black,  yellow  or  violet  masses  and,  like  the 
bacteria,  appear  to  be  dependent  to  a  degree  on  organic  material. 
At  least  they  are  especially  abundant  in  the  presence  of  decaying 
organic  matter,  as  in  the  drainage  from  stables  and  watering 
troughs,  or  in  pools,  puddles  and  damp  places  where  there  is  an 
abundance  of  filth  and  decay.     The  enormous  increase  of  these 
plants  often  produces  a  discoloration  of  the  water,  as  in  the  Red 
Sea,  and  they  are  often  the  cause  of  the  foul  odors  of  muddy 
ponds  and  streams  and  reservoirs  and  of  the  pollution  commonly 
known  as  water-bloom  or  working  of  ponds.     Like  the  bacteria 
also  they  are  associated  with  varying  amounts  of  gelatinous  sub- 
stances derived  from  their  walls  or  excreted  from  the  cells  and  in 
both  cases  the  walls  may  become  colored  with  various  pigments. 
Doubtless  the  mucilaginous  character  of  these  plants  enables 
them  to  retain  moisture  and  so  adapts  them  to  dryer  conditions 
than  would  otherwise  be  possible.     The  Cyanophyceae  differ 
radically  from  the  bacteria  in  possessing  chlorophyll  which  is  dis- 
tributed in  the  outer  portion  of  the  protoplasm,  and  in  the  pos- 
session of  a  more  definite  nucleus.    A  blue  pigment,  phycocyanine, 
is  associated  with  the  chlorophyll  and  for  this  reason  they  are 
called  the  blue-green  algae  or  Cyanophyceae.     Cell  division  fol- 
lows the  ordinary  method  noted  in  the  higher  plants,  at  least  this 
appears  to  be  true  in  those  cases  that  have  been  accurately 
studied.     These  plants  accordingly  show  a  decided  advance  over 
the  preceding  forms  not  only  in  the  differentiation  of  the  cells 
but  especially  because  they  are  capable,  to  a  degree  at  least,  of 
manufacturing  food  from  inorganic  substances  by  reason  of  their 
chlorophyll. 

In  addition  to  the  multiplication  of  individuals  by  cell  divi- 


166     REPRODUCTION   OF  THE   CYANOPHYCEAE 


sion,  as  in  the  case  of  the  bacteria,  many  of  the  thread-like  forms 
have  another  interesting  method  of  increasing  their  numbers. 
A  few  of  the  cells,  termed  a  hormogonium,  become  separated 


FIG.  92. 


FIG.  93. 


FIG.  92.  Forms  of  the  Cyanophyceae:  A,  Gleocapsa.  At  the  right  a  cell 
has  divided,  but  the  two  daughter  cells  are  held  together  in  a  gelatinous 
mass.  Above  numerous  divisions  have  occurred,  but  all  the  cells  are  sur- 
rounded by  a  mucilaginous  envelope.  B,  Nostoc.  Below  appear  the  gelat- 
inous, spherical  masses  of  the  plants  as  they  appear  floating  upon  the  sur- 
face or  resting  on  the  bottom  of  ponds.  At  the  right  a  plant  enlarged — 
h,  heterocyst;  s,  thick- walled  resting  cells  or  spores.  At  the  left  a  spore  has 
germinated,  producing  five  cells.  C,  Rivularia.  At  left  gelatinous  mass  of 
plants  attached  to  stem  of  water  plant.  At  right  view  of  a  few  of  the  plants 
— hr,  hormogonium;  h,  heterocyst. 

FIG.  93.  One  of  the  most  common  forms  of  the  Cyanophyceae,  Oscittatoria. 
The  different  sizes  of  the  cells  show  that  cell  division  may  occur  in  any  of 
the  cells  of  the  filament — e,  a  decaying  cell  which  will  ultimately  free  the 
cells  below  it  as  a  hormogonium. 


DEVELOPMENT   OF   PLANTS  167 

from  the  other  cells  and  frequently  possess  for  a  time  a  slight 
motility  that  enables  them  to  move  away  from  the  parent  fila- 
ment (Fig.  92,  hr).  In  some  species  one  or  several  rather  large, 
colorless  cells,  heterocysts,  appear  in  the  filament  and  may  serve 
to  separate  it  into  hormogonia  (Fig.  92,  h).  In  Oscillatoria 
(Fig.  93)  the  filaments  into  which  these  hormogonia  grow  per- 
manently retain  their  power  of  motion.  No  cilia  have  been 
observed  for  a  certainty  in  any  of  these  forms  and  the  cause  of 
the  motion  has  been  ascribed  to  the  interchange  of  fluids  attend- 
ant upon  the  absorption  and  digestion  of  foods.  A  simple  type 
of  spore  production  is  also  to  be  found  in  the  majority  of  forms. 
This  consists  merely  of  a  slight  enlargement  and  thickening  of 
the  walls  of  the  ordinary  cells  (Fig.  92,  s).  These  simple  spores 
are  able  to  tide  the  plant  over  unfavorable  conditions  and  germi- 
nate, forming  new  plants  when  conditions  are  again  suitable. 

The  Bacteria  and  Cyanophyceae  are  evidently  very  primitive 
and  ancient  forms  of  life.  The  fact  that  they  frequently  occur 
in  hot  springs  and  that  they  can  endure  greater  extremes  of 
heat  than  higher  plants  may  indicate  that  they  are  related  to 
forms  that  appeared  upon  the  earth  when  just  such  conditions 
existed  and  at  a  time  when  the  environment  was  not  suitable 
for  the  development  of  higher  types.  It  has  been  stated  on 
page  162  that  certain  bacteria  are  independent  of  organic  foods 
and  that  they  affect  the  decomposition  of  CO2  as  in  photosyn- 
thesis while  in  the  purple  and  in  the  red  sulphur  bacteria  we  have 
pigmented  forms  that  are  perhaps  in  a  transition  state  to  chloro- 
phyll-bearing plants.  It  seems  altogether  probable  that  the 
earliest  forms  of  life  could  build  up  organic  compounds  without 
chlorophyll  and  in  such  forms  as  these  bacteria  we  have  perhaps 
an  illustration  of  a  tendency  towards  the  acquisition  of  chloro- 
phyll as  seen  in  the  higher  plants. 

Subdivision  3.    Diatomaceae  or  Diatoms 

64.  The  Nature  and  Structure  of  Diatoms. — The  diatoms  are 

among  the  most  common  and  widely  distributed  plants  (Fig. 

94).     While  microscopic,  they  exist  in  such  large  numbers  as 

to  form  the  familiar  brown  coatings  on  the  bottom  of  ponds  and 


1 68 


FORMS   OF   DIATOMS 


sluggish  streams  and  often  render  sticks  and  stones  in  the  water 
slippery  with  slimy  deposits.  They  are  widely  distributed  on 
damp  soil  and  in  fresh  and  salt  waters.  Note  should  also  be 


FIG.  94.  Common  forms  of  diatoms:  A,  fan-shaped  colonies  of  diatoms, 
Licmophora,  attached  by  gelatinous  stalks  to  seaweed.  The  repeated  divi- 
sion of  a  solitary  diatom  gradually  builds  up  the  fan-like  colonies.  B,  Tabel- 
laria,  forming  colonies  grouped  in  zig-zag  blocks.  C,  Melosira,  cylindrical 
pill-box-like  diatoms,  arranged  in  chains.  Below  end  view  of  the  diatom. 
D,  a  species  of  Navicula  arranged  in  gelatinous  branches.  At  right  enlarged 
view  of  a  branch  in  which  the  diatoms  glide  back  and  forth. — After  Wm. 
Smith. 

made  of  the  peculiar  species  that  float  upon  the  surface  of  the 
ocean  and  form  the  larger  part  of  that  organic  life  known  as 


DEVELOPMENT   OF   PLANTS 


169 


the  plankton  (Fig.  95).  This  is  especially  abundant  in  northern 
waters  and  has  been  compared  to  great  pasture  lands,  since  it 
furnishes  the  principal  food  of  surface  feeding  fishes  and  other 
marine  life.  The  Diatoms  are  unicellular  plants  and  exhibit 


FIG.  95.  FIG.  96. 

FIG.  95.  Plankton  forms  of  diatoms:  A,  Coscinodiscus,  B,  Planktoniella. 
Below  seen  from  side. — After  Gran. 

FIG.  96.  Structure  of  the  diatom,  Pinnularia:  A,  valve  view.  B,  girdle 
view.  C,  cross-section,  the  two  dark  bands  showing  the  position  of  the  chro- 
moplasts  in  the  diatom — p,  pore  in  wall  which  appears  as  a  line  in  A ,  running 
from  the  ends  towards  the  center  of  the  valve. — After  Lauterborn. 

a  great  variety  of  forms  as  circular,  elliptical,  rod  or  wedge- 
shaped,  curved  or  straight.  They  are  equally  variable  as  regards 
their  association.  Some  are  solitary  and  free  swimming,  others 
are  attached  by  stalks;  some  form  bands  or  ribbons  or  zigzag 

12 


STRUCTURE   OF   DIATOMS 


chains,  while  others  are  imbedded  in  a  gelatinous  mass  of  a 
more  or  less  regular  form  (Fig.  94,  D).  The  cells  are  covered 
by  two  valves,  one  of  which  overlaps  the  other  like  the  cover 
of  a  box.  Therefore  a  diatom  presents  two  quite  distinct  ap- 
pearances— the  top  or  valve  view  and  the  side  or  girdle  view 
(Figs.  96,  97).  This  difference  is  further  intensified  by  the 
sculpturing  of  fine  lines  that  appear  upon  the  walls.  An  inter- 
esting feature  about  these  valves  is  the  fact  that  they  are  com- 


FIG.  97.  Structure  of  Cymbella:  A,  valve  view.  B,  cross-section  show- 
ing the  difference  of  the  two  sides  of  the  diatom.  C,  the  two  girdle  views. 
—After  Pfitzer. 

pletely  infiltrated  with  silica,  a  substance  resembling  glass.  If 
a  diatom  is  burned  in  a  flame  on  a  strip  of  platinum  or  placed 
in  acid  to  remove  the  organic  substance  the  appearance  of  the 
valves  remains  unchanged. 

The  glass-like  valves  are  quite  transparent  and  it  can  be  readily 
observed  that  the  cell  contents  is  much  more  highly  differenti- 
ated than  in  the  preceding  group.  The^chlorophyll  is  deposited 
in  pjastids  of  definite  form,  although  this  color  is  often  masked 
by  a  brown  pigment  which  causes  the  characteristic  appearance 
of  these  plants  when  associated  in  masses.  The  oil  drops  seen 
in  the  cells  are  the  product  of  photosynthesis — starch  not  being 
formed.  Some  species,  however,  can  live  upon  decaying  or- 
ganic matter  and  in  consequence  contain  colorless  plastids.  In 
the  free  swimming  forms  the  motion  consists  of  an  irregular 


DEVELOPMENT  OF   PLANTS  171 

gliding  movement  and  this  is  supposed  to  be  due  to  the  expan- 
sion and  contraction  of  minute  strands  of  protoplasm  that  pro- 
ject through  the  pores  of  the  valves  (Fig.  96,  C,  p) . 

(a)  Reproduction  of  the  Diatoms. — It  will  naturally  be  asked 
how  can  these  plants  living  in  glass  houses,  grow?  As  the  valves 
become  changed  to  silica  naturally  any  increase  in  size  must 
cease.  Nevertheless  the  cells  reproduce  with  great  rapidity  and 
in  a  very  interesting  manner.  Through  the  growth  of  the  living 
substance  the  valves  are  pushed  apart  and  the  cell  contents  di- 
vides, forming  two  diatoms  with  but  one  valve  each.  A  new 
valve  that  fits  into  the  old  valve  is  soon  developed  on  the  un- 
covered side  of  each  diatom  and  two  complete  diatoms  are  thus 
formed  (Fig.  98)  which  may  become  free  at  once  or  remain  at- 


FIG.  98.  Diagram  illustrating  the  divisions  of  a  diatom  and  the  resulting 
reduction  in  size.  The  brackets  connect  the  two  diatoms  that  were  formed 
by  each  successive  division. — After  Pfitzer. 

tached  and  by  further  division  form  the  odd  groupings  or  colo- 
nies shown  in  Fig.  94.  Since  the  valves  are  of  unequal  size,  i.  e.y 
one  fitting  into  the  other,  the  two  diatoms  formed  by  the  division 
must  vary  in  size  and  it  will  be  seen  that  the  majority  of  the  off- 
spring will  become  greatly  reduced  in  size  as  a  result  of  the 
rapid  and  repeated  divisions.  That  this  reduction  may  not  go 
on  too  far,  when  a  certain  minimum  size  has  been  reached  the 
cell  contents,  with  or  without  dividing,  throws  off  the  valves 
entirely  and  grows  to  the  full  size  of  the  diatom,  when  new 
valves  are  formed  and  the  diatom  is  ready  to  repeat  the  dividing 
process.  In  some  cases  we  find  quite  a  different  method  of  re- 
production. Two  diatoms  become  enclosed  in  a  jelly-like  mass, 
into  which  the  cell  contents,  with  or  without  dividing,  is  dis- 
charged. These  naked  cells  now  unite  in  pairs  and  the  bodies 
thus  formed  finally  grow;  into  diatoms  (Fig.  99).  This  latter 


172 


REPRODUCTION   OF   DIATOMS 


method  of  reproduction  is  termed  sexual  reproduction.  The  two 
fusing  cells  are  looked  upon  as  male  and  female  cells  or  gametes, 
although  in  these  simple  plants  there  is  no  indication  of  sex  that 
would  enable  us  to  recognize  them  as  such.  In  the  preceding 
groups,  multiplication  in  numbers  or  reproduction  has  been 
effected  by  the  division  of  the  cells  or  by  the  formation  of  more 
or  less  modified  cells,  termed  spores.  All  such  methods  of  repro- 
duction are  called  asexual,  since  but  one  cell  is  utilized  in  the 
process.  In  sexual  reproduction  a  cell  is  formed  by  the  union 
of  the  contents  of  two  cells.  The  nuclei  and  protoplasmic  con- 
tents of  each  unite  so  completely  that  a  cell  with  but  one  nucleus 
and  protoplasmic  contents  results.  These  sexually  produced 
cells  are  called  gametospores  because  they  are  formed  by  the 


B 


FIG.  99.  Sexual  reproduction  of  Pinnularia:  A  two  diatoms,  enveloped 
in  a  mass  of  jelly.  The  valves  have  been  thrown  off  and  the  content  of 
each  diatom  has  divided  into  two  sexual  cells  or  gametes.  B,  the  fusion  of 
the  gametes. — After  Karsten. 

union  of  two  gametes  and  are  capable  of  forming  a  new  plant 
in  the  same  way  as  do  spores.  We  may  distinguish  the  spores 
derived  from  single  cells  as  asexually  formed  spores  or  we  may 
call  them  simply  spores,  while  the  spores  formed  by  the  fusion 
of  the  two  cells  or  gametes  may  be  termed  sexually  formed  spores 
or  gametospores.  Both  kinds  of  spores  are  devices  to  enable 
the  plant  to  increase  in  number  or  to  meet  some  other  problem 


DEVELOPMENT   OF   PLANTS  173 

in  life,  such  as  conditions  unfavorable  for  growth.  The  spores 
are  especially  to  be  looked  upon  as  devices  to  bring  about  a  rapid 
increase  in  the  number  of  individuals  while  the  gametospores 
more  usually  serve  to  carry  the  plant  over  periods  of  drought  or 
extremes  of  temperature  which  would  prove  fatal  to  the  plant. 
For  this  reason  the  gametospores  are  often  provided  with  thick 
walls  and  dense  cell  contents  which  enable  them  to  remain  in  a 
dormant  state  until  conditions  are  favorable  for  growth.  The 
gametospore  is  often  called  a  resting  spore  for  this  reason. 

So  rapidly  do  the  diatoms  multiply  that  the  larger  part  of  the 
soil  and  rocks  in  many  localities  is  composed  of  their  remains. 
The  constant  casting  off  of  the  valves  through  reproduction  or 
death  results  in  vast  deposits,  known  as  silicious  earth,  on  the 
bottom  of  ponds  and  in  the  sea.  Beds  of  silicious  earth  formed  in 
this  way  are  often  seen  in  districts  where  ponds  and  lakes  have 
dried  up.  Sometimes  these  beds  reach  enormous  proportions, 
the  city  of  Richmond,  Va.,  being  built  upon  such  a  deposit. 
Some  of  the  western  beds  are  quite  300  feet  in  thickness  and  yet 
it  takes  about  40  million  of  these  plants  to  make  a  cubic  inch. 
Silicious  earth  is  used  as  polishing  powders  and  as  absorbents  in 
the  manufacture  of  some  explosives. 

Subdivision  4.    Euphyceae  or  Algae 

65.  General  Features. — TJiis  subdivision  includes  a  large  num- 
ber of  plants  that  live  chiefly  in  fresh  or  salt  water.  These  plants, 
popularly  known  as  Algae,  vary  greatly  in  form  and  structure 
and  range  from  microscopic  unicellular  forms  to  some  of  the  larg- 
est and  most  highly  constructed  plants  found  among  the  Thallo- 
phyta.  Their  advance  over  preceding  groups  appears  especially 
in  their  well-marked  walls  and  distinct  plastids  and  nuclei 
(Fig.  100,  104).  Chlorophyll  is  always  present  in  the  cells{ 
although  in  certain  groups  it  is  masked  by  brown  or  red  pigments. 
These  latter  pigments  are  supposed  to  adapt  the  plant  to  vary- 
ing intensities  of  light.  This  is  supported  by  the  fact  that  the 
marine  Algae  exhibit  a  zonal  distribution  in  the  water.  Forms 
living  near  the  surface  of  the  water  are  predominantly  green,  at 
depths  where  they  are  alternately  exposed  and  covered  by  the 


174  ORDERS   OF   GREEN   ALGAE 

tide,  they  are  more  commonly  brown,  while  in  the  shade  of  the 
brown  Algae,  or  below  tidal  limits,  red  forms  occur.  The 
Algae  appear  to  have  developed  along  three  lines,  which  are 
indicated  by  the  green,  brown  and  red  colors,  although  the  basis 
for  this  classification  rests  upon  structural  and  reproductive 
characters.  These  three  classes  are:  A,  Green  Algae  or  Chloro- 
phyceae;  B,  Brown  Algae  or  Phaeophyceae ;  C,  Red  Algae  or 
Rhodophyceae. 

CLASS  A.     GREEN  ALGAE  OR  CHLOROPHYCEAE 

66.  The  More  Important  Orders  of  the  Chlorophyceae. — The 

Chlorophyceae  are  one  of  the  most  interesting  groups  of  the 
Algae,  because  they  contain  primitive  forms  that  are  at  least 
suggestive  of  low  animal  types  and  they  also  exhibit  a  gradual 
modification  of  the  plant  body  -and  reproductive  processes  that 
help  us  to  understand  how  variations  arose  and  how  complex 
types  have  been  evolved  from  very  simple  forms.  These  plants 
are  doubtless  the  most  primitive  survivors  of  a  line  from  which 
the  higher  land  plants  have  been  derived.  Starting  with  uni- 
cellular motile  forms,  the  green  algae  appear  to  have  diverged 
along  several  lines.  Among  the  more  important  of  these  may  be 
mentioned  the  following  orders:  (a)  Volvocales,  largely  uni- 
cellular green  Algae;  (&)  The  Zygnematales  or  conjugating  green 
Algae;  (c)  Chaetophorales  or  filamentous  green  Algae;  (d) 
Siphonales  or  tubular  green  Algae. 

67.  Order  a.    Volvocales  or  Unicellular  Green  Algae. — The 
lower  members  of  this  order  represent  a  very  primitive  type  of 
plant.     Their  motility,  certain  features  of  their  life  history  and 
delicate  cell  walls  are  suggestive  of  some  of  the  preceding  groups, 
as  well  as  of  low  forms  of  animal  life.     Sphaerella  (Fig.  100)  is 
a  familiar  example  of  this  group,  often  appearing  in  rock  hollows 
as  blood -red  stains.     If  some  of  this  red  material  is  examined 
in  water,  the  plants  appear  as  spherical  cells  with  dense  red  pro- 
toplasmic contents  and  thick  walls  (Fig.   100,  A).     This  con- 
dition represents  the  resting  state  of  the  plant.     If  the  plants, 
after  being  dried,  are  allowed  to  stand  in  water  for  a  few  hours, 
the  nucleus  will  divide,  forming  usually  from  4  to  1 6  daughter 


DEVELOPMENT   OF   PLANTS 


175 


cells  (Fig.  100,  E)  which  escape  by  the  rupturing  of  the  old 
mother  wall  and  swim  actively  about  by  means  of  two  cilia. 
These  cells,  or  zoospores,  remain  motile  for  varying  periods, 
during  which  time  they  increase  in  size,  but  finally  the  cilia  are 


FIG.  100.  Stages  in  the  life  history  of  Sphaerella:  A,  resting  state  of  the 
plant.  B,  first  division.  C,  second  division,  the  four  cells  are  about  to 
escape  from  the  mother  plant.  D,  one  of  the  cells  of  C  after  escaping.  This 
is  a  zoospore  of  the  first  generation.  E,  zoospore  at  rest.  F,  forming  four 
new  zoospores.  G,  one  of  these  zoospores  of  the  second  generation.  Note 
that  the  red  material,  represented  by  the  shaded  area  in  the  center  of  the 
cell,  has  become  greatly  reduced  and  that  the  wall  is  becoming  distended  and 
separated  from  the  granular  cytoplasm.  H,  third  resting  stage.  I,  cell 
dividing.  K,  zoospore  of  third  generation  with  greatly  distended  cell  wall 
and  small  red  area.  Delicate  strands  of  cytoplasm  connect  the  cell  wall 
and  the  central  protoplasmic  body.  L,  a  resting  cell  dividing  into  a  large 
number  of  zoospores  which  are  consequently  smaller. — After  Hazen. 

retracted  and  the  plants  remain  in  a  quiescent  state  for  a  short 
period  (Fig.  100,  E).  A  division  of  the  nucleus  now  results 
again  in  the  formation  of  two  or  more  zoospores  (Fig.  100,  F) 
which  repeat  the  life  history  noted  above.  This  method  of 
reproduction  may  occur  again  and  again  and  thus  rapidly  bring 
about  a  great  increase  in  the  number  of  plants.  During  the 


176  LIFE   HISTORY   OF   SPHAERELLA 

divisions,  the  red  coloring  matter  is  usually  rapidly  replaced  by 
chlorophyll  until  but  a  red  speck  remains  and  the  zoospores  soon 
appear  as  rather  ovate  green  cells  surrounded  by  delicate  walls 
which  become  widely  separated  from  the  chlorophyll  owing  to 
the  accumulation  of  water  (Fig.  100,  G).  Nearly  all  zoospores 
that  occur  among  the  algae  are  characterized  by  a  small  red  body, 
known  as  the  eye  spot,  which  is  located  near  the  ciliated  end  of 
the  zoospore.  It  has  been  supposed  that  this  body  is  sensitive 
to  light  and  gives  the  zoospore  a  sense  of  direction.  While  this 
is  questionable,  since  the  eye  spot  may  be  lacking  (as  in  Sphae- 
rella), it  is  certain  that  these  beautiful  bodies  swim  about,  cili- 
ated end  foremost,  with  a  rotary  motion  from  right  to  left  and 
adjust  themselves  to  a  suitable  illumination.  It  can  easily  be 
demonstrated  that  the  zoospores  of  Sphaerella  are  keenly  sensi- 
tive to  different  intensities  of  light  by  placing  them  in  a  glass 
dish  by  a  window  when  the  zoospores  will  congregate  on  the 
illuminated  side  unless  the  light  is  too  intense,  when  a  reverse 
action  takes  place. 

Thus  we  see  that  the  life  of  the  plants  is  largely  a  motile  one, 
each  generation  being  characterized  by  a  short  resting  stage 
during  which  division  occurs  and  a  longer  motile  zoospore  stage. 
In  fact,  the  zoospore  may  be  looked  upon  as  the  original  state 
of  these  plants,  while  the  resting  condition  is  a  departure  due 
to  changes  in  the  environment  or  the  condition  of  the  organism. 
This  common  occurrence  of  motility  in  the  lower  types  of  life 
indicates  that  possibly  such  was  the  condition  of  the  first  life  upon 
the  earth. 

(a)  Conditions  Affecting  the  Life  of  Sphaerella. — Changes  in 
the  surroundings  sometimes  produce  remarkable  variations  in 
the  life  of  the  plant.  This  fact  is  well  illustrated  in  Sphaerella. 
If  the  water  dries  up,  a  thick  wall  is  formed  about  the  central 
protoplasmic  body  which  becomes  dense  and  of  a  deep  red  color 
while  the  delicate  distended  wall  and  cilia  disappear.  This  varia- 
tion adapts  the  plant  to  conditions  unfavorable  to  growth,  such 
as  drought  and  severe  temperature.  The  so-called  red  snow  is 
due  to  a  certain  species  of  these  plants  that  are  swept  off  from 
the  rocks  in  this  resting  condition  by  the  winter  winds  and  falling 


DEVELOPMENT   OF   PLANTS  177 

to  the  earth  produces  red  streaks  in  the  snow.  Hazen  has 
been  able  to  show  that  a  most  suggestive  change  in  the  mode  of 
life  of  the  plants  may  also  be  induced  by  low  temperature  or  by 
a  reduction  of  the  volume  of  water  in  which  they  live.  Under 
such  conditions,  bodies  are  formed  that  resemble  zoospores,  but 
that  are  not  capable  of  motion  owing  to  the  lack  of  cilia.  This 
helps  us  to  understand  how  the  motionless  or  stationary  forms 
of  plant  life  came  about  and  how  the  motile  stage  became  less 
and  less  conspicuous.  Owing  to  such  causes  as  these  and  many 
others,  plants  become  stationary.  At  first,  perhaps,  simply  an 
aggregate  of  motionless  cells,  but  finally  there  resulted  chains  or 
filaments  of  cells  owing  to  the  repeated  division  of  the  plants  in 
one  plane.  By  division  in  two  planes,  membranous  expansions 
resulted  and  the  more  complex  types  arose  by  the  division  of 
the  cells  in  three  planes  and  by  modification  of  the  cells. 

One  other  feature  appears  in  the  life  of  the  Sphaerella  that 
indicates  clearly  how  sexual  reproduction  came  about.  When 
the  conditions  for  growth  are  unfavorable,  as  for  example,  through 
lack  of  moisture  or  low  temperatures,  there  result  plants  that 
are  not  so  well  nourished,  or  at  least  they  appear  to  lack  some 
of  the  qualifications  that  characterize  the  plants  growing  under 
favorable  conditions.  The  same  condition  often  arises  in  these 
plants  after  several  generations  have  been  formed.  Such  plants, 
which  may  be  characterized  as  weaker,  or,  at  least,  lacking  in 
certain  material,  do  not  give  rise  to  the  ordinary  zoospores, 
but  to  much  smaller  though  similar  bodies  that  are  lacking  in 
cell  walls  (Fig.  100,  L).  These  small  zoospores  usually  perish 
or  produce  small  plants  unless  two  of  them  meet,  when  a  fusion 
may  occur  and  thus  a  plant  is  formed  that  is  capable  of  repeating 
the  life  history  of  Sphaerella.  By  suitable  nourishment,  how- 
ever, these  small  zoospores  may  sometimes  be  made  to  develop 
into  the  normal  plant.  It  is  very  evident  that  these  small  zoo- 
spores  are  lacking  in  some  substance  that  is  essential  to  their 
growth.  This  is  rarely  furnished  to  them  in  nature  except  when 
two  of  the  bodies  fuse.  From  numerous  examples  appearing 
among  different  groups  of  Algae,  it  appears  that  sexuality  arose 
in  this  way.  Owing  to  certain  conditions  of  light,  temperature, 


I78  ORIGIN   OF  SEX 

food,  etc.,  zopspores  were  formed  that  were  incapable  of  further 
growth.  But,  by  the  union  of  two  of  these  zoospores,  a  cell  was 
formed  possessed  of  renewed  vigor  and  capabilities  of  growth. 
So  we  can'  think  of  the  sexual  cells  or  gametes  as  zoSspores 
that  are  lacking  in  the  materials  essential  to  growth  and  of  the 
sexual  process  as  a  union  of  the  two  bodies  for  the  purpose  of 
bringing  together  the  missing  material  and  supplying  the  neces- 
sary energy  for  growth. 

A  closely  allied  genus,  Chlamydomonas  (Fig.  101)  has  a  life 
history  very  similar  to  that  of  Sphaerella  but  the  formation  of 
the  gametes  reveals  a  variation  that  gives  us  an  understanding 
of  how  these  bodies  came  to  differ  and  finally  became  distin- 
guishable as  male  and  female  gametes.  In  some  of  the  species 
of  Chlamydomonas,  to  be  sure,  the  gametes  are  similar  and  can 
not  be  distinguished  as  male  or  female  (Fig.  101,  B).  This 
stage  of  sexuality  is  called  isogamous,  meaning  similar  gametes. 
In  other  species,  there  is  a  decided  variation  in  the  size  of  the 
gametes,  certain  cells  or  plants  producing  large  and  less  active 
cells  that  are  termed  female  gametes,  while  other  plants  produce 
numerous  minute  male  gametes  that  swim  actively  about  (Fig. 
101,  C,  D).  This  stage  of  sexuality,  where  the  gametes  differ 
in  size  is  called  heterogamous,  meaning  unequal  gametes.  This 
variation  in  the  size  of  the  gametes  which  we  call  the  differentia- 
tion of  sex  is  due  to  the  amount  and  nature  of  the  material  stored 
in  them,  the  male  gametes  being  formed  in  a  cell  in  larger  num- 
bers than  in  the  case  of  the  females.  These  heterogamous 
gametes  do  not  differ  in  nature  from  the  isogamous  gametes. 
They  are  also  lacking  in  some  substance  that  renders  them  in- 
capable of  growth  unless  fusion  of  two  gametes  is  effected. 

(b)  Colonial  Unicellular  Algae. — The  various  stages  in  the 
evolution  of  sex  are  also  well  illustrated  in  several  genera  of 
unicellular  green  algae  that  live  together  in  colonies.  These 
colonial  forms  are  also  of  interest  in  that  they  help  us  to  under- 
stand how  unicellular  plants  became  associated — at  first  quite 
independent  of  one  another — and  this  led  to  the  dependence  of 
one  plant  upon  another  and  finally  to  a  distribution  of  work 
among  the  plants  of  the  colony,  so  that  certain  plants  or  cells 


DEVELOPMENT  OF   PLANTS 


179 


would  be  largely  concerned  in  reproduction  and  others  in  the 
manufacture  of  foods,  etc.  This  is  essentially  the  state  reached 
by  our  higher  plants  which  may  be  spoken  of  as  a  colony  of  cells. 
Pandorina  shows  the  earlier  phases  of  the  differentiation  of  the 
gametes.  This  plant  is  really  a  spherical  colony  of  about  16,  prac- 


FIG.  101. 


FIG.  102. 


FIG.  101.  Features  in  the  life  history  of  Chlamydomonas:  A,  character 
of  the  motile  plant.  B,  conjugation  of  isogamous  gametes.  C,  a  plant 
dividing  to  form  numerous  small  male  gametes.  D,  a  plant  forming  two  large 
female  gametes.  E,  male  and  female  gametes  about  to  conjugate. 

FIG.  102.  Features  in  the  life  history  of  Pandorina:  A,  a  colony  of  plants. 
B,  each  plant  of  the  colony  dividing  to  form  a  new  colony.  C,  the  plants  of 
a  colony  escaping  as  gametes.  D,  the  conjugation  of  two  gametes  of  un- 
equal size.  E,  later  stage  in  the  conjugation.  F,  gametospore  or  resting 
spore.  G,  large  zoospore  formed  from  the  gametospore.  H,  a  colony  formed 
by  the  division  of  the  zoospore,  G. 

tically  independent,  motile  plants,  each  one  similar  to  Sphaerella 
but  held  together  in  a  mucilaginous  mass  (Fig.  102,  A).  These 
beautiful  colonies  multiply  by  each  plant  or  cell  dividing  into  16 
daughter  cells  which  finally  become  an  independent  colony 
(Fig.  102,  B).  The  gametes  are  formed  in  essentially  the  same 
way  as  the  colonies,  save  that  the  16  daughter  cells  finally  escape 
as  free  swimming  bodies  (Fig.  102,  C).  The  mother  plants  or 
cells  producing  these  gametes  differ  in  size,  owing,  doubtless,  to 


i8o  DIFFERENTIATION   OF  SEX 

the  amount  of  nutriment  which  they  receive.  Consequently, 
the  gametes  formed  from  them  must  also  differ  in  size.  It  is 
also  possible  that  the  mother  plants  may  produce  gametes  of 
different  sizes  because  some  divide  a  larger  number  of  times  than 
others.  The  larger  the  number  of  divisions  in  a  cell,  the  smaller 
will  be  the  gametes.  It  is  interesting  to  note  that  these  gametes 
fuse  quite  regardless  of  their  size,  although  there  may  be  a  slight 
tendency  for  the  small  and  more  active  gametes  to  fuse  with 
the  larger  and  slower  moving  ones.  Thus  we  have  an  illustra- 
tion of  the  first  appearance  of  the  differentiation  of  sex  although 
the  sexuality  of  the  gametes  is  not  thoroughly  established.  The 
gametospore,  resulting  from  the  union  of  the  gametes,  develops 
a  thick  cell  wall,  the  contents  assumes  a  reddish  color  and  it 
passes  into  a  resting  stage  (Fig.  102,  F).  When  conditions  are 
favorable,  it  germinates  as  shown  in  Fig.  102,  G-H.  In  the 
beautiful  spherical  colonies  of  Eudorina  and  Vokox  (Fig.  103) 
the  differentiation  of  the  gametes  is  complete.  Certain  cells 
produce  but  one  large  female  gamete  that  remains  motionless 
in  the  mother  cell  while  other  cells  form  numerous  small  motile 
male  gametes  which  are  yellowish  in  color.  In  Volvox,  over 
200  male  gametes  may  be  formed  from  a  single  cell  (Fig.  103, 
D,  E).  Vokox  also  shows  that  the  association  of  plants  in 
colonies  resulted  in  an  interdependence  among  the  plants  and 
also  in  a  specialization  in  the  work  performed  by  these  plants. 
Fig.  103,  C,  shows  that  the  thousands  of  plants  that  make  up  the 
colony  are  in  communication  with  one  another  and  they  doubt- 
less give  and  receive  material  in  accordance  with  the  needs  of 
the  colony.  Furthermore  a  few  of  the  cells  of  the  colony  receive 
more  nourishment  than  the  others  and  these  cells  increase  in  size, 
divide  and  form  new  colonies  as  in  Pandorina.  Finally  there  is 
to  be  noticed  that  certain  cells  function  in  the  production  of 
gametes.  So  we  see  here  in  a  simple  way  some  of  those  speci- 
alizations and  distributions  of  labor  that  will  appear  as  the  most 
characteristic  features  in  higher  forms. 

The  tendency  among  the  algae  to  form  gametes  of  different 
sizes  is  manifestly  of  great  advantage.  The  great  number  of 
male  cells  increases  the  chances  of  fusion  of  the  gametes  and  their 


DEVELOPMENT   OF   PLANTS 


181 


smallness  promotes  their  movements  through  the  water.  Note 
also  that  these  results  are  obtained  without  any  additional 
expenditure  of  material — the  200  male  gametes  being  produced 
from  a  cell  no  larger  than  Sphaerella  where  only  a  few  are  found. 
The  increase  in  the  size  of  the  female  gamete  renders  her  move- 
ments more  difficult,  but  the  storage  of  food  in  this  gamete  pro- 
vides for  the  better  nourishment  of  the  next  generation.  Finally 
the  nourishing  function  becomes  so  strongly  developed  that  the 
female  ceases  to  move  at  all  and  remains  protected  in  the  mother 


B 


FIG.  103.  Features  in  the  life  history  of  Volvox:  A,  a  colony  which  may 
contain  as  many  as  20,000  plants.  The  three  central  groups  are  young  col- 
onies which  may  arise  by  the  repeated  division  of  any  of  the  plants.  B,  side 
view  of  one  of  the  plants  showing  the  canal-like  connections  with  the  neigh- 
boring plants.  C,  surface  view  of  a  plant  with  canals  radiating  out  to  the 
adjoining  plants.  D,  a  plant  enlarging  and  forming  a  single  motionless  female 
gamete.  E,  a  plant  forming  numerous  small  male  gametes.  F,  male  gametes 
enlarged. 

cell.  Thus  we  see  that  sexuality  may  have  arisen  among  plants 
as  the  result  of  definite  stimuli  as  heat,  moisture,  light,  food  and 
other  conditions  that  occur  in  the  environment.  These  stimuli 
produce  enfeebled  zoospores  that  are  incapable  ordinarily  of 
growth  unless  a  fusion  of  two  of  them  is  effected.  So  sexuality 


1 82 


MOTIONLESS   UNICELLULAR  ALGAE 


arose  from  the  asexual  state  of  the  plant  as  a  result  of  the  influence 
of  the  environment  and  differentiation  of  sex  appeared  owing  to 
the  variation  in  the  amount  and  kind  of  material  in  the  gamete 
producing  cells. 

(c)  Motionless  Unicellular  Algae. — In  contrast  to  these  motile 
plants,  there  is  a  large  group  of  the  Volvocales  that  have  lost 
their  motility  and  spend  a  large  part  of  their  life  as  stationary 
plants.  We  have  already  noticed  how  this  condition  may  have 


FIG.  104.  Common  forms  of  stationary  unicellular  green  algae:  A,  Pleu- 
rococcus  showing  stages  in  the  division  of  the  plants,  the  nucleus  and  the 
irregular  chloroplnst.  B,  Scenedesmus.  Below  three  of  the  plants  are  shown 
dividing.  C,  Ankistrodesmus.  At  right  three  plants  in  process  of  division. 

come  about,  owing  to  the  temperature  and  moisture  conditions 
that  were  unfavorable  for  the  motile  existence.  Pleurococcus 
(Fig.  104,  A}  is  one  of  the  most  common  of  the  green  algae 
and  illustrates  many  of  the  features  in  the  life  history  of  the 
stationary  forms.  These  plants  often  cause  the  green  coating 
that  appears  upon  the  shaded  or  moist  side  of  tree  trunks, 
fences  and  buildings.  If  a  bit  of  this  material  is  examined  under 


DEVELOPMENT   OF   PLANTS  183 

the  microscope,  it  will  be  seen  that  the  plants  are  rather  spherical 
cells  provided  with  nucleus,  cytoplasm,  chloroplasts  and  well 
marked  walls.  This  latter  feature  is  characteristic  of  stationary 
algae  and  forms  a  sharp  contrast  to  the  delicate  walls  of  the 
motile  forms.  These  plants  increase  in  numbers  by  the  forma- 
tion of  a  wall  through  the  middle  of  the  cell.  The  two  daughter 
cells  or  plants  grow  to  the  size  of  the  mother  plant  and  repeat  the 
process  in  division  by  forming  a  wall  at  right  angles  to  the  old 
wall.  It  will  also  be  noticed  that  cell  division  may  occur  in 
three  planes.  The  plants  adhere  together  for  a  considerable 
period,  or  become  detached  shortly  after  the  division  of  the 
mother  cell.  In  this  way  they  multiply  with  extreme  rapidity 
and  so  come  to  form  extensive  green  coatings.  It  has  been 
noted  in  allied  genera  and  reported  in  certain  species  of  Pleuro- 
coccus  also,  that  changes  of  temperature  and  amount  of  water, 
etc.,  will  cause  the  contents  of  the  cell  to  divide  and  form  zoo- 
spores  which  are  of  the  same  nature  and  behave  in  the  same  way 
as  in  Sphaerella.  Thus  we  see  how  changes  of  the  environment 
may  cause  these  stationary  plants  to  return  to  the  motile  condi- 
tion of  simpler  types.  It  is  interesting  to  note  in  this  connection 
that  Livingston  was  able  to  change  the  stationary  condition  of 
certain  algae  to  the  motile  condition  at  will  by  diluting  the 
solutions  in  which  they  were  living.'  Fig.  104,  B,  C,  illustrates 
several  other  forms  of  stationary  green  algae  that  are  common 
in  the  drinking  water  and  in  ponds  and  streams.  Interesting 
forms  for  study  may  be  obtained  by  tying  a  woolen  sack  to 
a  water  faucet  and  allowing  the  water  to  run  into  it  for  several 
hours.  Invert  the  sack  into  a  glass,  washing  off  the  material  that 
collects  on  the  inside  of  the  sack  in  the  water  of  the  glass.  By 
means  of  a  pipette,  a  drop  of  the  material  that  collects  at  the 
bottom  or  sides  of  the  glass  or  forms  on  the  surface  of  the  water 
may  be  transferred  to  a  slide  and  examined  under  a  microscope. 
Some  of  these  forms  are  free,  some  attached  and  others  aggre- 
gated in  a  mucilaginous  mass. 

There  are  a  great  many  of  the  stationary  genera  that  show  the 
same  habit  of  uniting  into  colonies  that  we  noticed  among  the 
motile  genera.  These  colonies  may  be  very  simple  and  com- 


1 84 


HYDRODICTYON 


posed  of  a  few  loosely  aggregated  plants  forming  flat  discs  or 
star-shaped  plates  (Fig.  105,  A).  In  other  cases,  we  find  spher- 
ical and  net-like  colonies.  The  water  net,  Hydrodictyon,  is  a 
common  illustration  of  this  latter  arrangement  (Fig.  105,  D,  E). 
These  plants  occur  in  ponds  and  sluggish  streams,  forming  sac- 
like  nets  from  a  few  inches  to  a  foot  in  length,  that  are  constructed 


FIG.  105.  Colonial  forms  of  unicellular  green  algae:  A,  Pediastrum,  the 
plants  of  the  colony  being  arranged  in  a  flat  plate.  B,  a  view  of  the  outer 
cells  of  the  colony  showing  the  formation  of  a  new  colony.  C,  one  of  these 
new  colonies.  D,  a  plant  of  the  water  net  containing  a  young  colony.  E, 
enlarged  view  of  one  of  the  meshes  of  a  net  showing  the  geometrical  arrange- 
ment of  the  plants. 

in  a  very  regular  manner.  The  reproduction  is  quite  character- 
istic of  many  of  the  Volvocales.  Zoospores  are  formed  in  large 
numbers  in  any  of  the  plants  and  after  a  period  of  motility,  they 
come  to  rest  and  arrange  themselves  into  a  new  net  inside  of  the 
mother  plant  (Fig.  105,  D).  The  walls  of  the  mother  plant  soon 
break  down  and  the  new  colony  is  set  free.  Under  different 
conditions,  smaller  but  similar  bodies  are  formed  which  escape 
from  the  mother  plant.  These  bodies  are  gametes  and  incapable 
of  growth  unless  a  fusion  is  effected  between  two  of  them. 


DEVELOPMENT   OF   PLANTS 


185 


Especial  interest  attaches  to  the  study  of  this  plant  because 
Klebs  was  able  to  change  the  reproductive  character  of  the  plant 
by  subjecting  it  to  certain  definite  conditions.  By  exposing  the 
plants  to  bright  light,  slight  increase  of  temperature  and  in- 
organic culture  solutions  of  definite  composition,  zoospores  are 
produced.  On  the  other  hand,  plants  growing  in  solution  of 
cane  sugar  in  subdued  light  or  darkness  formed  gametes. 

These  reactions  are  of  great  importance  in  helping  us  to  under- 
stand the  nature  of  plant  life.  They  demonstrate  that  definite 
factors  may  change  motile  plants  to  a  stationary  condition  and 
in  Hydrodictyon  we  see  that  measurable  factors,  as  light,  heat 
and  concentration  of  the  solution  may  bring  about  the  motile 
condition  of  the  plant  and  may  also  cause  a  variation  in  the 
motile  condition  which  we  call  sexuality. 

68.  Order  b.  Zygnematales  or  Conjugating  Green  Algae. — 
This  order  includes  the  most  common  and  attractive  forms  of 
green  algae  (Figs.  106,  108).  They  are  of  very  general  occur- 


FIG.    1 06.     Two   common   forms   of   the   Zygnematales:   A,   Spirogyra — 
n,  nucleus;  p,  pyrenoid.     B,  Zygnema. 

rence  in  the  still  waters  of  ponds  and  streams,  where  they  often 
form  the  large  floating  and  frothy  green  masses  popularly  known 
as  pond  scum  or  frog  spittle.  They  can  usually  be  recognized 
by  their  slimy  nature  which  is  due  to  the  copious  exudation  of  a 
gelatinous  substance  from  the  cell  walls.  In  one  group  of  this 
order  the  plants  are  multi-cellular  (Fig.  106),  the  cells  being 
attached  end  to  end  and  forming  a  filament — though  it  should  be 
stated  that  each  cell  is  practically  independent  in  its  life  process 
13 


186  REPRODUCTION   OF  SPIROGYRA 

and  the  organism  may  therefore  be  regarded  as  a  colony.  Such 
a  type  of  plant  would  have  originated  in  the  case  of  Pleurococcus 
if  the  two  daughter  cells  had  remained  attached  and  continued 
to  divide  in  but  one  plane  and  parallel  to  the  first  division  wall. 
This  is  exactly  the  mode  of  growth  in  these  filamentous  plants. 
Any  of  the  cells  may  divide  into  two  daughter  cells  which  grow 
to  the  original  size  of  the  mother  cells.  In  this  way  the  length 
of  the  plant  is  increased.  The  most  characteristic  feature  about 
the  Zygnematales  is  the  remarkably  large  and  attractive  chloro- 
plasts.  In  Spirogym  (Fig.  106,  A)  these  appear  as  bands,  from 
one  to  several  in  each  cell,  spirally  arranged  just  within  the  cell 
wall.  In  Zygnema  (Fig.  106,  B)  they  assume  the  form  of  stars 
and  in  other  genera  they  appear  as  plates,  bars  and  variously 
modified  bodies.  Imbedded  in  the  plastids  are  denser  proto- 
plasmic bodies,  the  pyrenoids,  that  appear  to  be  connected  with 
the  secretion  of  starch ;  at  least  a  layer  of  starch  can  be  detected 
about  the  pyrenoids  by  testing  with  iodine  (Fig.  106,  pr). 

The  filaments  are  fragile  and  readily  become  dissociated  into 
smaller  portions.  In  this  way,  the  plants  may  multiply,  since  the 
fragments  continue  to  grow  after  the  manner  of  the  original 
filament.  Singularly,  there  is  no  indication  of  a  return  to  the 
motile  condition  in  the  life  history  of  these  plants.  No  zoospores 
or  motile  gametes  are  formed.  Sometimes  the  walls  of  a  cell 
become  thickened  and  after  a  dormant  period,  as  through  the 
winter,  may  germinate  and  grow  into  a  new  plant.  With  this 
rather  rare  exception,  the  asexual  reproduction  of  the  plant  by 
spores  does  not  occur.  The  sexual  reproduction  is  of  a  peculiar 
character.  This  is  very  well  illustrated  in  Spirogyra.  When 
filaments  of  this  plant  chance  to  lie  side  by  side,  under  certain 
conditions,  tube-like  processes  from  opposite  cells  begin  to  grow 
out  towards  each  other  (Fig.  107,  A).  These  tubes  finally  meet, 
when  the  walls  at  the  point  of  contact  are  absorbed,  forming  an 
open  passage  between  the  two  cells.  While  this  growth  has  been 
going  on,  the  contents  of  the  two  cells  have  contracted  some- 
what and  finally  the  contents  of  one  cell  passes  through  the  tube 
and  fuses  with  the  contents  of  the  other  cell.  Several  modi- 
fications of  the  reproduction  process  outlined  above  will  occa- 


DEVELOPMENT   OF   PLANTS 


187 


sionally  be  noted,  such  as  the  development  of  gametospores  in 
either  filament  or  the  fusion  of  the  adjacent  cells  of  a  filament  by 
means  of  lateral  tubular  outgrowth.  In  other  genera,  there  is  a 
considerable  modification  in  the  process,  as  in  some  species  of 
Zygnema,  where  the  fusion  is  effected  in  the  middle  of  the  tube. 


FIG.  107.  Sexual  reproduction  of  Spirogyra:  A,  in  lower  portion  of  figure 
the  formation  of  the  tubes  between  the  opposite  cells  of  the  filaments  is  shown. 
Above  the  contents  of  two  cells  have  united  with  two  cells  of  another  filament, 
forming  two  gametospores.  B,  germination  of  a  gametospore. 

This  is  only  a  different  form  of  the  sexual  process  with  which 
you  are  familiar,  the  difference  being,  that  but  one  gamete  is 
formed  in  each  cell  and  these  bodies  are  motionless  or  at  least 
not  provided  with  cilia  and  do  not  escape  from  the  mother  plant. 
When  the  fusion  of  the  gametes  has  been  effected  the  resulting 


188       GERMINATION   OF  THE   GAMETOSPORE 

gametospore  becomes  surrounded  by  a  thick  wall  and  in  this 
resting  condition  it  tides  the  plant  over  seasons,  such  as  the 
winter  or  drought,  unfavorable  for  the  growth  of  the  plant. 
When  conditions  return  that  permit  the  growth  of  the  plant, 
the  gametospore  germinates  by  rupturing  the  outer  wall  and  pro- 
truding the  inner  wall  of  the  spore  as  a  delicate  tube  (Fig.  107,  B) . 
Early  in  this  germination  the  nucleus  divides,  forming  four  cells, 
the  outermost  of  which  only  continues  to  grow  and  so  ultimately 
forms  the  new  plant.  Note  that  the  gametospore,  therefore, 


FIG.  108.  Common  forms  of  Desmids:  A,  Closterium.  B,  Micr aster ias , 
C,  Xanthidium.  D,  Staurastrum.  At  left  top  view,  at  right  side  view.  £, 
Desmidium  forming  a  chain  of  plants.  At  right,  end  view  of  chain. 

does  not  grow  directly  into  the  parent  type  of  plant.  This  be- 
havior of  the  gametospore  is  one  of  the  most  fundamental 
features  in  plant  life.  It  appears  in  the  preceding  groups  but 
it  is  mentioned  now  for  the  first  time  because  we  see  here  a  clear 
demonstration  of  its  character.  The  gametospore  is  different 
in  its  nature  and  possibilities  of  growth  from  any  one  of  the  cells 
of  the  parent  plant.  It  can  not  produce  the  spirogyra  plant. 
It  can  only  produce  four  cells  and  from  one  of  these  cells  there 
will  develop  the  spirogyra  plant.  Further  consideration  of  this 
feature  in  the  life  history  of  the  plant  will  be  deferred  to  later 


DEVELOPMENT   OF   PLANTS 


189 


examples,  where  it  will  become  still  more  obvious.  The  loss  of 
motility  in  the  reproductive  process  in  this  order  may  possibly 
be  associated  with  their  exposure  to  terrestrial  conditions. 
They  are  often  exposed  to  the  soil,  owing  to  the  drying  up  of  the 
water  and  it  has  been  suggested  that  they  have  consequently  lost 
their  motility.  Their  mucilaginous  coatings  would  be  of  great 
service  in  enabling  them  to  meet  such  conditions  by  retaining 
moisture,  and  indeed,  they  are  often  able  to  flourish  in  many 
damp  places  without  the  aid  of  surface  water. 

The  Desmids. — A  second  group  of  the  Zygnematales  are  strictly 
unicellular  plants  known  as  Desmids  (Fig.  108).  They  are  the 
most  attractive  of  unicellular  plants  and  are  of  common  occur- 
rence associated  with  coarse  algae.  The  desmids  are  elaborately 
and  variously  fashioned  but  can  readily  be  recognized  by  the 


FIG.  109.  Reproduction  of  Cosmarium:  A,  a  sexual  reproduction,  show- 
ing the  elongation  of  the  isthmus,  i,  and  its  gradual  enlargement  and  divi- 
sion to  form  the  new  lobes  of  the  desmids.  B,  sexual  reproduction.  At 
the  left  the  cell  contents  of  two  desmids  fusing  to  form  a  gametospore.  On 
the  right  the  mature  gametospore  covered  with  a  spiny  coat. 

fact  that  they  consist  of  two  similar  halves  (Fig.  109,  A).  The 
structure  of  the  cells  and  the  sexual  method  of  reproduction 
is  essentially  the  same  as  in  the  filamentous  forms  which  have 
doubtless  given  rise  to  these  unicellular  plants.  The  asexual 


190  SPECIALIZATION   IN   ULOTHRIX 

method  of  reproduction,  however,  is  rather  peculiar.  The  region 
connecting  the  two  halves  (Fig.  109,  A,  i),  called  the  isthmus, 
becomes  somewhat  elongated  and  swollen.  Soon  a  constriction 
appears  midway  between  these  two  halves  which  deepens  until 
the  desmid  is  cut  in  half.  Each  desmid  now  consists  of  one  of 
the  original  halves  and  one  half  of  the  isthmus.  This  latter 
part  gradually  enlarges  and  forms  the  second  half  of  the  desmid. 
Owing  to  the  gelatinous  character  of  the  cell  walls,  the  desmids 
often  become  aggregated  in  masses  and  form  chains  of  cells 
(Fig.  108,  E). 

69.  Order  c.  Chaetophorales  or  Filamentous  Green  Algae. — 
Here  belong  a  great  array  of  green  algae  that  grow  attached  to 
various  objects  and  appear  as  simple  or  more  usually  as  branched 
filaments  which  frequently  end  in  hair-like  tips.  They  are  of 
common  occurrence  at  the  bottom  or  margins  of  springs,  wells, 
streams  and  ponds.  Ulothrix  may  be  taken  as  an  example  of 
one  of  the  simple  forms  of  the  Chaetophorales.  Fig.  no  shows 
that  it  consists  of  a  simple  chain  of  cells  that  divide  and  so  bring 
about  the  growth  of  the  plant  as  in  Spirogym.  However,  it  is 
important  to  note  that  we  have  here  a  higher  type  of  vegetative 
structure  than  in  any  of  the  algae  heretofore  studied.  This 
advance  appears  in  the  one  or  more  rather  colorless  cells  at  the 
base  of  the  plant  which  serve  to  anchor  it  to  the  substratum.  In 
other  words,  there  is  apparent  a  distribution  of  labor  or  the  per- 
formance of  definite  duties  by  special  parts  or  cells  of  the  plant 
body.  The  evolution  of  higher  forms  comes  about  in  much  this 
way.  So  long  as  each  cell  of  the  plant  performs  the  same  func- 
tion, there  can  be  little  variation  in  the  form  and  structure.  But 
as  these  cells  are  differently  stimulated  by  their  environment, 
they  tend  to  vary  in  character  and  so  become  adapted  to  the  per- 
formance of  different  functions.  Perhaps  in  Ulothrix  the  con- 
tact of  the  substratum  acted  as  a  stimulus,  as  Pierce  found  to 
be  the  case  in  many  algae.  Thus  the  plant  comes  to  consist  of 
different  kinds  of  cells,  each  of  which  performs  a  different  kind 
of  work.  The  complexity  of  the  plant  is  due  to  the  variations  of 
its  cells  which  are  thereby  adapted  to  the  performance  of  special 
duties.  In  Ulothrix,  we  have  one  of  the  simplest  modifications 


DEVELOPMENT   OF   PLANTS 


191 


of  this  nature,  one  or  more  of  the  basal  cells  becoming  slightly 
changed  in  form  and  contents  and  so  adapted  in  anchoring  the 
plant.  The  other  cells  of  the  plant  are  practically  alike,  each  con- 
taining a  single  nucleus  and  chloroplast  and  therefore  capable  of 
manufacturing  food  and  forming  the  reproductive  bodies  which 
are  of  the  same  simple  type  as  noted  in  Sphaerella. 

(a)  Reproduction  of   Ulothrix. — The  contents  of  any  of  the 
green  cells  may  divide  into  two  or  more  cells  which  escape  as 


FIG.  no. 


FIG.  in. 


FIG.  no.  Lower  portion  of  Ulothrix,  the  basal  cells  are  somewhat  modi- 
fied and  the  lowest  one  acts  as  an  anchoring  organ.  Each  of  the  upper  cells 
contains  a  girdle-like  chloroplast. 

FIG.  in.  Asexual  reproduction  of  Ulothrix:  A,  a  few  cells  of  a  fila- 
ment in  the  upper  cells  of  which  the  formation  and  escape  of  the  large  zoo- 
spores  are  shown,  while  in  the  lowest  cells  a  large  number  of  small  zoospores 
appear.  B,  a  large  zoospore.  C,  a  young  plant  formed  by  B.  D,  a  small 
zoospore.  E,  a  young  plant  formed  from  D. — After  West.* 

zoospores  through  an  opening  formed  in  the  cell  wall  (Fig.  in, 
A).     The  cells  in  which  the  zoospores  are  formed  are  called 


192       ASEXUAL  REPRODUCTION  OF  ULOTHRIX 

sporangia.  The  zoospores  resemble  the  motile  plants  noted 
above,  but  after  a  short  motile  period  they  return  to  the  station- 
ary condition  and  grow  into  new  plants  (Fig.  HI,  C).  In  this 
way,  the  number  of  plants  is  rapidly  multiplied.  The  point 
must  not  be  overlooked  that  this  phase  of  the  life  history  is  the 
same  as  that  of  Sphaerella,  the  difference  appearing  in  the 
shortening  of  the  motile  period  and  in  the  lengthening  of  the 
non -motile  period.  During  the  resting  stage  the  cells  of  Ulothrix 
to  be  sure  may  divide  in  one  plane  forming  a  filament  but  re- 
member also  that  non-motile  cells  of  Ulothrix  may  divide,  forming 
a  mass  of  cells  that  become  detached  or  loosely  associated — the 
so-called  palmelloid  state — which  behavior  we  have  seen  sug- 
gested in  the  non-motile  cells  of  Sphaerella.  It  should  be  stated 
that  the  asexual  method  of  reproduction  may  be  looked  upon 
as  a  means  of  bringing  about  a  rapid  increase  in  the  number 
of  individuals  of  a  species  while  the  conditions  for  growth  are 
favorable.  When  new  conditions  arise,  such  as  changes  of  tem- 
perature, food,  etc.,  that  would  interfere  with  the  normal  growth 
of  the  individuals  then  a  modification  of  the  contents  of  the  cells 
and  its  mode  of  division  results.  'The  cells  of  Ulothrix  as  a  con- 
sequence of  such  alterations  divide  into  a  larger  number  of  motile 
bodies  which  are  consequently  smaller  than  in  the  case  of  zoo- 
spores  (Fig.  112,  A).  These  bodies  may  behave  as  gametes  and 
unite,  forming  gametospores.  The  cells  in  which  gametes  are 
formed  are  called  gametangia,  sing,  gametangium.  Thus,  Ulo- 
thrix is  controlled  in  the  same  manner  as  Chlamydomonas  and 
Hydrodictyon.  It  is  interesting  to  note,  however,  that  these 
small  bodies  frequently  behave  as  zoospores  and  develop  into 
small  and  weak  plants,  which  fact  accounts  for  the  common  asso- 
ciation in  this  genus  of  puny  and  vigorous  plants  (Fig.  HI,  C,  E). 
These  small  bodies  of  Ulothrix  represent  a  curious  intermediate 
condition  between  a  zoospore  and  a  gamete  where  the  sexual 
character  is  not  strongly  enough  developed  to  overcome  com- 
pletely the  zoospore  character.  Thus,  we  see  again  in  Ulothrix 
that  definite  environmental  conditions  caused  motile  bodies  to  be 
produced  that  are  lacking  in  some  essential  substance  and  conse- 
quently do  not  have  the  energy  for  growth,  or  at  least  can  only 


DEVELOPMENT   OF   PLANTS 


193 


develop  into  feeble  plants.  This  weakness  is  overcome  by  the 
fusion  of  two  gametes  which  results  in  the  formation  of  a  cell 
or  gametospore  with  renewed  energy  for  growth.  We  also  note 
how  these  departures  in  the  behavior  of  the  organism  are  of 
advantage  to  it.  The  zocspores  effect  a  distribution  of  the 
species  while  the  gametospore  ensures  a  continuity  of  the  species 
by  tiding  it  over  unfavorable  conditions. 

The  gametospore  germinates  after  a  period  of  rest  somewhat 
as  in  the  case  of  Spirogyra.  The  contents  of  the  spore,  instead 
of  giving  rise  directly  to  a  new  plant,  divides,  forming  several 
zoospores  (Fig.  112,  E,  F)  which  after  a  period  of  motility 


FIG.  112.  Sexual  reproduction  of  Ulothrix:  A,  a  few  cells  of  a  filament 
showing  the  formation  and  escape  of  the  gametes.  B,  gamete.  C,  D,  stages 
in  the  union  of  the  gametes.  E,  gametospore.  F,  gametospore  germin- 
ating and  forming  the  sporophyte  or  asexual  generation,  which  is  a  sac-like 
plant  containing  several  zoospores. 

become  attached  to  some  object  and  grow  into  a  new  plant. 
The  cause  of  this  peculiar  behavior  of  the  germinating  gameto- 
spore is  not  known.  Some  change  has  occurred  in  it  that 
renders  it  incapable  of  developing  directly  into  a  new  plant. 
This  variation  in  the  growth  of  the  gametospore  marks  the 
beginning  of  one  of  the  most  important  departures  in  the  evo- 
lution of  plant  life  and  it  will  become  more  and  more  significant 
as  the  work  proceeds.  It  is  evident  that  the  formation  of 


194  NATURE   OF  THE   GAMETOSPORE 

several  zoospores,  each  of  which  produces  a  new  plant,  is  a  de- 
cided advance  over  those  types  in  which  the  gametospore  develops 
directly  into  but  one  new  plant.  Note  also  that  in  Ulothrix  there 
are  two  kinds  of  zoospores,  one  formed  from  the  Ulothrix  plant 
and  the  other  from  the  gametospore,  the  former  being  frequently 
called  zoogonidia  to  distinguish  them  from  the  latter.  This  dis- 
tinction is  of  great  importance,  because  most  plants  have  two 
distinct  stages  or  generations  in  their  life  history.  One  is  repre- 
sented by  the  plant  that  bears  the  sexual  cells  or  gametes  and 
therefore  called  the  sexual  plant  or  sexual  generation,  or,  in  short, 
the  gametophyte.  The  real  nature  of  this  plant  is  to  produce 
gametes,  although  it  may  for  a  time  produce  zoospores.  The 
other  stage  is  derived  from  the  gametospore  and  is  called  the 
asexual  plant,  asexual  generation  or  sporophyte,  since  this  plant 
can  only  produce  zoospores.  Among  the  green  algae  this  dis- 
tinction is  not  very  manifest,  the  Ulothrix  plant  for  example  being 
the  sexual  plant  or  gametophyte  and  the  gametospore  or  the 
little  plant  derived  from  it  (Fig.  112,  F)  is  the  asexual  plant  or 
sporophyte,  in  this  case  being  merely  a  single  cell.  It  is  very 
manifest,  however,  that  this  single  cell  or  plant  is  decidedly 
different  from  the  Ulothrix  plant  because  it  cannot  develop  into 
the  Ulothrix  plant,  but  can  only  produce  zoospores.  So  we  see 
that  there  are  two  generations  in  the  life  history  and  that  they 
differ  radically  in  their  natures  and  possibilities  of  growth.  In 
higher  types  of  plant  life  it  will  be  seen  that  the  gametospore 
tends  to  develop  into  a  more  and  more  complex  body  or  plant 
and  it  will  be  one  of  the  most  intresting  features  of  the  work  to 
watch  the  evolution  of  this  asexual  plant. 

The  higher  members  of  the  Chaetophorales  illustrate  the  same 
gradual  differentiation  of  the  gametes,  as  has  been  noted  in  the 
motile  forms  of  the  Volvocales.  The  female  gametes  become 
distinguishable  because  of  their  larger  size  and  shorter  period  of 
motility,  while  the  male  gametes  are  small,  owing  to  the  large 
numbers  that  are  found  in  a  cell  or  because  of  the  small  size  of 
the  cells  in  which  they  are  developed.  Finally,  in  several  of  the 
genera  we  find  but  a  single  female  gamete  which  remains  motion- 
less in  the  cell.  This  condition  is  very  well  illustrated  in  one  of 
the  genera  of  the  next  order. 


DEVELOPMENT   OF   PLANTS 


195 


70.  Order   d.     Siphonales   or   Tubular   Green   Algae.— This 

order  includes  a  large  number  of  odd  forms  that  are  filamentous 
in  character  and  they  differ  from  algae  previously  noted  in  that 
the  filaments  contain  numerous  nuclei,  but  with  rare  exceptions 
no  cell  partitions.  Such  plants  are  called  coenocytes.  They 
assume  various  forms  and  often  resemble  a  small  plant  with  stem, 
root  and  leaf,  but  in  all  these  cases  the  plant  is  essentially  a  huge 
cell  or  tube  without  partitions  and  containing  numerous  nuclei. 
Most  of  the  Siphonales  are  marine,  but  one  genus,  Vaucheria,  is 
well  represented  in  shallow  streams,  damp  places  and  on  the 
earth  of  flower  pots  in  greenhouses,  where  it  forms  rather  coarse 
green  felt-4ike  masses.  The  plant  body  consists  of  long  tubular 
threads,  often  branching  and  anchored  to  the  ground  by  colorless 


FIG.  113.  Structure  and  asexual  reproduction  of  Vaucheria:  A,  portion 
of  a  plant  showing  the  branching  tubular  filament  and  colorless  root-like 
outgrowth,  r,  B,  end  of  a  filament  enlarging  to  form  a  zoospore.  C,  zoospore. 
D,  germination  of  a  zoospore. 

outgrowths  (Fig.  113,  A).  The  protoplasm  forms  a  thick  lining 
layer  on  the  inner  wall  of  the  filament  and  embedded  in  it  are 
numerous  minute  chloroplasts,  nuclei  and  oil  drops.  A  watery 
cell  sap  fills  the  center  of  the  tube.  Partitions  only  appear  when 
reproductive  bodies  are  formed  or  to  close  a  wound  in  case  of 


196      SEXUAL   REPRODUCTION   OF  VAUCHERIA 

injury  to  the  filaments.  Asexual  reproduction  is  effected  by  very 
large  zoospores  which  are  developed  in  the  enlarged  extremities  of 
the  filaments  (Fig.  1 13,  B) .  These  tips  are  cut  off  by  a  transverse 
wall  and  a  single  zoospore  escapes  through  an  opening  in  the 
tip  of  the  sporangium  thus  formed.  The  entire  surface  of  the 
zoospore  is  clothed  with  cilia  arranged  in  pairs,  each  pair  being 
associated  with  a  nucleus  so  that  the  zoospore  resembles  the 
motile  colonies  previously  noted  (Fig.  113,  C).  After  a  very 
short  motile  period,  the  zoospore  comes  to  rest  and  grows  into 
the  characteristic  tubular  plant  (Fig.  113,  D).  When  the  plants 
are  exposed  to  too  dry  conditions,  the  tips  of  the  filaments  often 


FIG.  114.  Sexual  reproduction  of  Vaucheria:  A,  portion  of  a  filament  that 
has  formed  two  branches  which  have  grown  into  a  male,  an,  and  female,  og, 
gametangia.  B,  later  stage,  the  gametangia  have  opened,  permitting  the 
escape  of  the  male  gametes  and  the  fertilization  of  female  gamete.  C,  gam- 
etospore  detached  from  the  filament. 

enlarge  and  finally  become  detached  as  motionless  spores  that 
germinate  when  conditions  are  again  favorable. 

The  male  and  female  gametes  are  produced  in  gametangia  that 
are  formed  from  short  branches.  The  males  are  developed  in 
large  numbers  in  curved  branches  that  become  cut  off  from  the 
filament  by  a  cross  wall  (Fig.  114,  A)  and  the  gametes  finally 
escape  through  an  opening  that  forms  at  the  apex  of  the  branch. 
A  gametangium  that  produces  clearly  differentiated  male  gametes 
is  called  an  antheridium  (plu.  antheridia)  and  the  gametes  are 
frequently  called  antherozoids  or  sperms.  A  single  female 


DEVELOPMENT   OF   PLANTS 


197 


gamete  is  found  in  a  similar  branch  which  becomes  rather  egg- 
shaped  and  at  maturity  opens  at  the  beaked  end  (Fig.  114,  B). 
A  single-celled  female  gametangium  is  called  an  oogonium  and 
the  female  gamete  is  frequently  referred  to  as  the  oosphere  or 
egg.  The  male  gametes  are  discharged  a  few  minutes  after  the 
oogonium  opens,  when  they  swarm  about  the  open  end  of  the 
female  gametangium  and  readily  enter  it.  As  soon  as  fusion  has 
been  effected  the  gametospore  becomes  invested  with  a  thick  wall 
and  in  this  condition  can  endure  a  limited  drought.  In  germi- 
nating it  develops  directly  into  a  new  plant.  This  feature  of  the 


FIG.  115.  Growth  and  asexual  reproduction  of  Oedogonium:  A,  young 
plant  showing  basal  cell  modified  as  a  hold-fast.  B,  two  cells  of  a  filament, 
in  one  of  which  a  zoospore  is  forming  and  the  other  cell  has  opened,  per- 
mitting the  escape  of  the  zoospore  shown  at  C.  D,  zoospore  at  rest.  It 
becomes  attached  to  an  object  at  its  colorless,  ciliated  end.  E,  later  stage 
in  the  germination  of  the  zoospore. — After  West. 

life  history  of  Vaucheria,  therefore,  appears  as  of  a  more  primitive 
nature  than  in  the  case  of  Ulothrix.  In  the  following  studies  you 
will  repeatedly  notice  variations  of  this  nature.  In  the  develop- 
ment of  plants  advances  are  often  confined  to  one  or  another 
feature  of  their  organism  which  may  become  highly  developed 
and  specialized;  at  the  same  time  one  or  more  features  may  be 
retained  that  have  been  subject  to  little  or  no  variation  and 
which,  therefore,  remain  in  a  primitive  state. 

71.  Other  Members  of  the  Green  Algae. — There  are  several 


198 


HIGHER   TYPES   OF   REPRODUCTION 


orders  of  the  Chlorophyceae,  some  of  which  are  marine,  that 
cannot  be  considered  at  this  time.  Two  genera,  however,  de- 
serve attention  because  they  show  significant  advances  in  the 
evolution  of  plant  life.  In  Oedogonium,  a  member  of  a  small 
order,  the  Oedogoniales,  we  find  a  still  higher  form  of  the  sexual 
reproductive  process.  These  plants  are  of  very  common  occur- 
rence in  ditches,  streams  and  springs  where  their  filaments  form 
for  a  time  greenish  masses  and  coatings  upon  various  objects, 
but  finally  become  detached  and  free-floating  (Fig.  115,  A). 
Any  of  the  cells  of  a  filament,  save  the  basal  one,  may  form  a 
single  zoospore,  which  are  large  pear-shaped  bodies  with  a  narrow 


FIG.  116.  Sexual  reproduction  of  Oedogonium:  A,  portion  of  a  filament 
in  which  a  female  gamete  has  been  formed — o,  opening  in  cell  wall  for  en- 
trance of  male  gamete.  B,  portion  of  filament  showing  formation  and  es- 
cape of  male  gametes.  C,  gametospore  free  from  mother  cell.  The  germi- 
nation of  this  spore  results  in  the  rupture  of  its  outer  wall  and  the  protrusion 
of  the  contents  of  the  spore  as  four  cells  which  are  for  a  time  retained  by  the 
delicate  inner  wall  of  the  spore,  as  shown  in  D.  E,  the  four  cells  have  become 
mobile  zoospores  and  the  delicate  inner  wall  of  the  gametospore  is  greatly  dis- 
tended and  about  to  rupture.  F,  G,  stages  in  the  germination  of  the  zoospore. 
—After  Him.  . 

colorless  end  around  the  base  of  which  arise  a  circle  of  numerous 
cilia  (Fig.  115,  B,  C).  These  zoospores  develop  into  new  plants 
(Fig.  115,  D,  E)  as  in  previous  cases.  The  sexual  reproduction 
presents  a  high  degree  of  specialization.  The  male  gametes  are 


DEVELOPMENT   OF   PLANTS 


199 


usually  formed  in  pairs  in  small  cells  and  are  similar  to  but 
smaller  than  the  zoospores,  while  the  female  gametes  are  de- 
veloped singly  in  the  ordinary  cells  of  a  filament  which  become 
greatly  enlarged  and  spherical  or  void  in  form  (Fig.  116,  A,  B). 
The  female  gamete  is  motionless  but  the  colorless  region  on  one 
side  of  the  gamete  certainly  suggests  the  idea  that  this  body  is 


FIG.  117.  FIG.  1 1 8. 

FIG.  117.  Coleochaete,  showing  the  radiating  filaments  and  hair-like  out- 
growths of  the  cells. 

FIG.  1 1 8.  Sexual  reproduction  of  Coleochaete:  A,  end  of  a  filament  bearing 
the  male,  an,  and  female,  og,  reproductive  organs.  B,  the  gametospore  is 
being  enveloped  by  the  adjacent  filaments  of  the  plant.  C,  gametospore 
germinating.  Eight  cells  have  been  formed,  rupturing  the  walls  of  the  gameto- 
spore.— After  Oltmann. 

related  to  the  zoospores  and  male  gametes,  but  that  it  has  lost 
its  cilia.  At  maturity,  openings  of  various  kinds  appear  in  the 
oogonium  through  which  the  male  gametes  enter  and  one  fuses 
with  the  female  (Fig.  116,  A,  0).  The  thick-walled  gametospore 
that  results  from  this  union  is  set  free  by  the  decay  of  the  oogonial 
walls  and  germinates  after  a  resting  period  very  much  as  in  the 


200         ADVANCED  TYPE   OF  REPRODUCTION 

case  of  Ulothrix.  The  contents  of  the  gametospore  is  extruded 
through  the  ruptured  wall  as  a  delicate  sac  and  then  divides, 
forming  four  zoospores  which  produce  new  plants  (Fig.  116,  C-G) 
— thus  we  see  again  an  illustration  of  the  peculiar  nature  of  the 
gametospore  as  in  Spirogyra  and  Ulothrix.  The  question  will 
naturally  arise  as  to  the  force  that  brings  these  and  other  gametes 
together.  The  frequently  observed  aggregation  or  swarming  of 
the  male  gametes  about  the  females  renders  probable  the  view 
that  some  substances,  as  organic  acids,  etc.,  are  developed  in  the 
gametes  that  serve  to  draw  them  together  whenever  they  come 
within  a  certain  distance  of  each  other. 

The  consideration  of  Coleochaete,  a  plant  of  the  same  order  as 
Ulothrix,  has  been  deferred  to  this  point  because  it  presents 
several  interesting  departures  from  previous  types  which  indi- 
cate that  a  higher  point  has  been  attained  in  some  respects  by 
this  plant  than  by  any  other  of  the  green  algae.  The  filaments  of 
Coleochaete  have  a  pronounced  apical  growth  and  are  usually 
associated  together  in  a  radiate  manner,  forming  small  discs  or 
cushion-like  masses  on  the  stems  and  leaves  of  water  plants  (Fig. 
117).  The  advance  of  this  type  is  indicated  not  only  by  the 
localization  of  growth  at  definite  points,  but  also  by  the  formation 
of  the  zoospores  and  gametes  in  definite  regions,  i.  e.,  in  special 
cells  which  are  usually  located  at  the  end  of  the  filaments.  The 
zoospores  are  produced  singly  from  such  cells,  and  from  smaller 
pear-shaped  cells  single  male  gametes  are  formed.  The  female 
gametes  are  developed  singly  in  large  flask-shaped  cells,  access 
to  which  is  afforded  by  an  opening  that  appears  at  the  end  of 
the  long  neck  of  the  flask  (Fig.  118,  A).  The  gametospore  de- 
velops a  cell  wall  and  becomes  enveloped  by  the  adjoining  cells 
of  the  filaments  (Fig.  118,  B).  In  this  condition  the  winter  is 
passed  and  in  the  spring  it  germinates,  forming  neither  a  plant 
like  the  parent  type  nor  zoospores  as  in  Ulothrix  and  Oedogonium, 
but  instead,  the  gametospore  forms  a  number  of  cells.  This 
growth  ruptures  the  coat  of  the  spore  and  finally  from  each  cell  a 
rather  irregular  zoospore  (Fig.  118,  c)  is  derived  that  develops 
into  a  small  plant.  The  plants  thus  formed  multiply  solely  by 
zoospores  until  finally,  after  several  generations,  larger  plants 


DEVELOPMENT   OF   PLANTS  201 

are    produced    that    bear  gametes,    thus    completing    the  life 
history. 

72.  Noteworthy  Features  of  the  Chlorophyceae. — Two  features 
in  the  study  of  green  algae  should  be  kept  clearly  in  mind, 
because  they  are  closely  connected  with  the  tendencies  that  will 
appear  in  the  development  of  the  mosses.     First:  The  game  to- 
spore  is  essentially  a  dormant  or  resting  cell  that  tides  the  life 
of  the  plant  over  the  conditions  unfavorable  for  growth.     Second : 
In  passing  from  lower  to  higher  types,  the  gametospore  tends 
to  vary  in  its  nature  and  possibilities  of  growth.     At  first  it 
forms  directly  a  sexual  or  gamete-bearing  plant  as  in  Sphaerella 
but  in  higher  types,  it  develops  a  generation  of  zoospores  from 
which  the  sexual  plant  is  derived  as  in  Ulothrix  and  Oedogonium. 
Finally  in  Coleochaete,  we  find  a  further  advance  in  that  several 
non-motile  cells  are  formed  by  the  gametospore.     These  cells, 
to  be  sure,  are  essentially  like  the  zoospore  because  they  develop 
directly  into  zoospores,  but  this  condition  emphasizes  the  fact 
that  the  nature  of  the  gametospore  is  steadily  departing  from 
that  of  the  gamete-bearing  plant. 

This  development  could  be  summarized  by  saying  that  there 
is  a  tendency  to  separate  the  life  history  of  the  plant  into  two 
stages ;  a  sexual  or  gametophyte  generation  and  an  asexual  sporo- 
phyte  generation.  These  two  generations  follow  each  other  in 
the  life  history  of  all  the  higher  plants.  This  peculiar  relation- 
ship is  called  the  alternation  of  generations  and  the  reason  for 
it  will  be  more  apparent  when  we  come  to  study  the  mosses. 

CLASS  B.    THE  BROWN  ALGAE  OR  PHAEOPHYCEAE 

73.  General  Features. — With  few  exceptions  the  Phaeophyceae 
are  marine  plants.     They  are  commonly  known  as  brown  algae, 
owing  to  the  brownish  pigments  which  conceal  in  a  measure 
the  chlorophyll,  thus  producing  their  characteristic  brown  or 
yellow   color.     They  are  of  common  occurrence   along   rocky 
shores  and  attain  enormous  dimensions  in  the  northern  and 
southern  seas  and  on  the  Pacific  Coast.     As  a  rule,  the  plant 
body  is  better  differentiated  than  in  the  green  algae  and  shows 
a  higher  type  of  specialization  than  yet  seen.     It  would  appear 

14 


202  SIMPLER   BROWN   ALGAE 

probable  that  the  brown  algae  have  been  derived  from  green 
algae  but  there  is  no  satisfactory  evidence  of  a  series  of  unicellu- 
lar forms  leading  up  to  the  rather  complex  types  that  are  repre- 
sentative of  the  simplest  of  the  Phaeophyceae.  The  majority 
of  these  lower  forms  have  already  reached  the  filamentous  stage 
and  many  of  the  higher  genera  exhibit  a  differentiation  of  the 
plant  body  suggestive  of  the  higher  plants,  as  for  example,  a 
definite  axis  with  branches  and  leaf-like  outgrowths  and  root- 
like  organs  that  anchor  the  plants  to  the  substratum.  Likewise, 
the  tissues  of  these  higher  types  often  reveal  many  of  the  features 
already  noted  in  the  epidermis,  cortex  and  central  region  of 
terrestrial  plants.  The  life  history  of  the  brown  algae  indicates 
that  they  have  undergone  the  same  evolution  as  the  Chloro- 
phyceae.  For  purposes  of  comparison  we  will  consider  only 
two  groups  of  the  brown  algae. 

(a)  The  Simpler  Brown  Algae. — Ectocarpus  may  be  taken  as 
an  example  of  this  group.  It  is  a  filamentous  branching  form 
that  is  almost  universally  distributed  along  the  sea  shore  (Fig. 
119).  The  zoospores  are  developed  in  certain  cells  of  the  upper 
branches.  It  is  important  to  note  in  the  brown  algae,  that 
the  reproductive  cells  are  generally  confined  to  special  parts 
of  the  plant  and  not  promiscuously  developed  as  in  the  green 
algae.  This  marks  a  decided  advance  in  the  evolution  of  the 
plant.  The  tissues  are  becoming  more  specialized  and  the  work 
performed  by  the  plant  is  apportioned  to  special  cells  and  organs. 
The  zoospores  have  essentially  the  same  structure  and  mode  of 
growing  into  new  plants  as  noted  in  Ulothrix.  The  cilia,  how- 
ever, are  laterally  attached  and  this  is  a  characteristic  feature 
of  the  zoospores  of  the  Phaeophyceae  generally  (Fig.  119,  A,  g). 

The  sexual  reproduction  of  Ectocarpus  exhibits  some  of  the 
most  instructive  variations  to  be  found  in  all  the  algae.  The 
gametes  are  formed  in  gametangia  that  become  divided  into 
great  numbers  of  very  small  cubical  cells,  each  one  of  which 
produces  a  single  gamete  (Fig.  119,  B).  Thus  the  origin  of 
the  gametes  is  more  specialized  than  in  the  case  of  the  green 
algae  where  numerous  gametes  were  formed  in  a  single  cell. 
You  are  to  remember  this  feature  of  the  gametangium  because 


DEVELOPMENT   OF   PLANTS 


203 


some  of  its  characteristics  will  reappear  in  Bryophyta.  The 
gametes  themselves,  however,  show  the  same  range  of  vari- 
ations as  previously  noted.  In  the  simpler  types  of  this  genus 
they  are  quite  alike  and  on  the  same  plane  of  differentiation  as 


.A 


FIG.  119.  Reproduction  of  Ectocarpus:  A,  branch  bearing  several  spo- 
rangia, sp;  g,  zoospore  enlarged.  B,  branch  bearing  gametangia,  gm.  C,  the 
upper  motionless  gamete  (female)  surrounded  by  several  still  motile  gametes 
(males).  Dt  fusion  of  the  gametes,  below  the  resulting  gametospore. 

in  Ulothrix,  since  they  sometimes  develop  as  zoospores,  directly 
forming  the  plant,  or  they  may  behave  as  gametes  and  fuse  to 


204     SEXUAL   REPRODUCTION   OF   ECTOCARPUS 

form  a  gametospore.  The  nature  and  character  of  these  bodies  is 
so  imperfectly  established  that  their  behavior  is  entirely  deter- 
mined by  external  conditions.  Low  temperatures  and  bright 
light  tend  to  develop  these  bodies  as  zoospores,  whereas,  high 
temperatures  cause  them  to  behave  as  gametes.  (See  Pandorina 
and  Hydrodictyon.)  Other  species  of  Ectocarpus  show  an  ad- 
vance over  this  stage.  The  gametes  are  alike,  but  it  has  been 
observed  that  certain  ones,  the  female  gametes,  have  a  shorter 
period  of  motility  and  after  coming  to  rest  they  attract  the  still 
motile  gametes,  the  males,  and  cause  one  to  fuse  with  them 
(Fig.  119,  C,  D).  This  is  the  simplest  distinction  that  can  be 
pointed  to  as  indicating  a  difference  in  the  nature  of  the  gametes 
which  we  call  sex.  This  variation  in  the  period  of  motility  of 
the  gametes  must  be  due  to  an  essential  difference  in  the  material 
or  substance  of  which  they  are  composed,  although  there  is  no 
external  evidence  of  this.  Certain  species  of  this  same  genus 
also  reveal  a  variation  in  the  size  of  the  cells  from  which  the 
gametes  are  derived.  In  these  species,  the  sex  is  clearly  indi- 
cated by  the  larger  size  of  the  female  cell  and  it  is  noticeable 
that  there  is  a  decided  tendency  for  the  smaller  gametes  to  fuse 
with  the  larger  and  more  slowly  moving  females.  It  is  also 
worthy  of  note  as  these  gametes  become  differentiated  and  their 
sex  more  evident  that  they  are  less  able  to  behave  as  zoospores 
and  grow  directly  into  new  plants.  When  the  gametes  produced 
by  the  plant  are  all  alike  they  may  readily  be  made  to  grow  into 
new  plants,  but  this  is  rarely  the  case  where  they  can  be  dis- 
tinguished as  male  and  female.  Thus,  in  Ectocarpus,  we  have 
a  most  remarkable  series  of  variations  that  indicate  how  sexuality 
has  arisen  from  the  asexual  condition  and  also  how  sex  finally 
became  characterized  by  a  shorter  motile  period  in  the  female 
gamete.  The  character  of  the  female  finally  became  more  pro- 
nounced, owing  to  its  better  nourishment  and  consequent  increase 
in  size  and  slower  movements.  A  more  perfect  illustration  of 
the  evolution  of  sex  is  not  found  in  nature. 

The  differentiation  of  reproductive  parts,  as  will  be  frequently 
noted,  does  not  keep  pace  necessarily  with  the  evolution  of  the 
plant  body.  It  is  evident  that  sexuality  has  arisen  independently 


DEVELOPMENT  OF  PLANTS 


205 


at  different  levels  in  the  plant  world  and  in  groups  in  no  wise 
connected.  We  have  noted  the  same  state  of  sexuality  in  some 
of  the  motile  green  algae,  i.  e.,  in  Chlamydomonas  and  in  non- 
motile  forms  as  Hydrodictyon  and  Ulothrix,  and  now  again  in 
Ectocarpus  although  these  plants  are  widely  separated  as  far  as 
relationship  is  concerned  and  exhibit  a  marked  variation  in  the 
development  of  the  plant  body.  It  is  also  frequently  to  be 
noted  that  plants  may  get  along  very  well  indeed  with  only  a 
sexual  or  an  asexual  method  of  reproduction. 

(V)  The  Coarser  Brown  Algae,  the  Kelps. — Under  this  head 
we  may  consider  two  groups,  illustrated  by  the  kelps  and  rock- 
weeds.  The  kelps  are  related  to  Ectocarpus  and  include  the 
largest  and  most  highly  organized  forms  of  all  the  algae.  Indeed 
some  of  these  forms  are  quite  comparable  in  size  with  our  shrubs 
and  trees.  The  Laminarias  (Fig.  120,  A}  of  our  Atlantic  coast 


FIG.  1 20.     Three  of  the  larger  brown  algae:  A,  Laminaria.     B.  Lessonia. 

C,  Macrocystis. 

have  stalked  blades  ten  to  twenty  feet  long.  The  great  bladder 
kelps  of  the  Pacific,  Nereocystis  and  Macrocystis,  attain  great 
dimensions,  the  latter  genus  reaching  a  length  of  500  to  900  feet 
and  Lessonia  with  trunk-like  stems  and  leaf-like  segments  forms 
veritable  submerged  forests  in  the  Antarctic  Ocean  (Fig.  120, 
B,  C).  These  highly  organized  plants,  however,  do  not  show 
any  advance  in  sexual  reproduction  over  the  simplest  form  noted 
in  Ectocarpus.  Much  attention  has  been  directed  to  the  kelps 


206 


COARSER  BROWN  ALGAE 


as  a  source  of  potassium  (see  page  48),  from  23  to  48  per  cent, 
of  their  dry  weight  being  potassium  chloride.  It  is  estimated 
that  there  is  between  600  and  800  sq.  miles  of  these  plants  on  our 
Pacific  coast  and  that  they  should  yield  annually  one  million 
tons  of  potassium  chloride,  valued  at  40  million  dollars. 

The  rock  weed  or  bladder  wrack  (Fucus)  and  the  gulf  weed 
(Sargassum)  are  representatives  of  the  second  group,  Fucaceae, 
that  contain  the  most  specialized  of  the  brown  algae  (Fig.  121). 
The  Sargassum  with  its  stem  and  leaf-like  organs  which  may 
become  modified  into  air  sacs  and  reproductive  organs  bears  the 
closest  external  resemblance  to  the  higher  plants  of  any  of  the 
algae  (Fig.  121,  A).  It  forms  the  major  portion  of  that  floating 


FIG.  121.  Two  common  forms  of  the  Fucaceae:  A,  Sargassum,  the  stem- 
like  axis  bearing  air  sacs,  s,  and  leaf-like  organs;  g,  reproductive  branches.  B, 
Fucus — s,  air  sacs;  g,  reproductive  branch.  C,  young  plant. 

vegetation  in  the  Atlantic  known  as  the  Sargasso  Sea.  The 
bladder  wracks  may  be  found  firmly  attached  to  the  rocks  by 
disc-like  holdfasts  in  almost  all  colder,  temperate  and  northern 
seas  (Fig.  121,  B).  The  elongation  of  the  flat  leathery  stems 
is  largely  localized  in  a  terminal  cell  and  results  in  a  regular 


DEVELOPMENT   OF   PLANTS 


207 


forking  of  the  stem  into  two  equal  parts,  a  method  of  branching 
called  dichotomy  in  contradistinction  to  the  axial  branching 
characteristic  of  the  majority  of  our  flowering  plants.  Fucus, 
like  many  of  the  gross  brown  algae,  contains  air  cavities  or 
bladders  which  buoy  it  up  in  the  water;  this  feature  accounts 
for  its  popular  name  of  bladder  wrack.  A  cross  section  of  the 
stem  shows  that  the  tissues  of  these  plants  have  attained  a  con- 
siderable differentiation  (Fig.  122,  A)  as  is  attested  by  a  rudi- 


FIG.  122.  Structural  features  of  Fucus:  A,  cross-section  of  a  portion  of 
the  central  stem-like  part  of  the  plant,  showing  an  epidermal,  e,  cortical, 
cr,  and  central  region,  c.  B,  section  of  one  of  the  cavities  that  appears  to 
the  eye  as  a  dot.  See  Fig.  121,  B,  g.  This  cavity  contains  only  male  game- 
tangia.  C,  section  of  a  cavity  from  another  plant  contains  only  female  game- 
tangia. — After  Oltmann. 

mentary  epidermal,  cortical  and  central  region,  the  latter  often 
containing  well-marked  sieve  tubes.  These  elongated  cells  of 
the  central  region  promote  the  rapid  distribution  of  materials 
and  doubtless  account  in  part  for  the  size  obtained  by  the  kelps 
and  rockweeds. 


208  SEXUAL   REPRODUCTION   OF   FUCUS 

The  most  characteristic  feature  of  these  plants,  however,  and 
the  one  separating  them  sharply  from  the  Laminarias  appears 
in  their  method  of  reproduction.  None  of  the  Fucaceae  develop 
zoospores,  although  an  asexual  reproduction  may  be  effected  by 
the  detachment  of  small  branches  from  the  plants.  Sexual  re- 
production is  a  step  in  advance  of  any  of  the  preceding  types 
of  the  brown  algae,  in  that  the  female  gamete  becomes  still 
larger  and  loses  entirely  the  power  of  motion.  The  reproductive 
organs,  or  gametangia,  are  developed  in  specialized  branches  or 
enlarged  tips  of  the  thallus.  In  Fucus  (Fig.  121,  g),  these  organs 
are  contained  in  small  pits  or  cavities  that  appear  as  minute 
points,  or  as  dots  when  the  enlarged  tip  of  a  branch  is  held  up 
to  the  light.  A  magnified  section  taken  through  such  a  branch 
shows  the  nature  of  the  cavities  (Fig.  122,  B,  C).  In  some 
species,  the  male  and  female  gametangia  are  found  in  the  same 
cavity,  or  they  may  occur  separately  and  on  different  plants. 
The  male  gametes  are  developed  in  enormous  numbers  in  numer- 
ous little  sacs,  or  antheridia,  borne  on  branching  filaments  of  cells 


FIG.  123.  One  of  the  branching  filaments  from  Fig.  122,  B,  greatly  en- 
larged. Some  of  the  antheridia,  an,  have  discharged  the  male  gametes,  which 
are  still  retained  in  the  inner  wall  of  the  antheridium,  as  at  b.  At  a  this  wall 
is  ruptured,  freeing  the  gametes. 

which  grow  out  from  the  sides  of  the  cavities  (Fig.  123).  The 
female  gametes  are  produced  in  larger  sacs  or  oogonia,  which 
are  each  supported  on  a  single  cell  and  associated  with  hair-like 
chains  of  cells,  paraphyses  (Fig.  124,  A).  Usually  eight  gametes 
are  developed  in  each  oogonium.  The  gametes  when  first  dis- 
charged from  the  antheridia  and  oogonia  are  enclosed  in  a  delicate 


DEVELOPMENT   OF   PLANTS 


209 


cell  wall  which  later  ruptures  and  sets  them  free  (Fig.  124,  B). 
The  female  gametes  are  without  cilia  and  as  they  float  away  in 
the  water  they  appear  to  attract  the  male  gametes  which  swarm 
about  them  and  finally  fertilization  is  effected  by  one  of  the  male 
gametes  working  down  to  the  nucleus  of  the  female  and  fusing 
with  it  (Fig.  124,  C}.  In  an  allied  form  not  only  is  an  attractive 
substance  formed  in  the  female  gamete,  but  probably  a  repellent 


FIG.  124.  Female  gametangia:  A,  greatly  enlarged  view  of  one  of  the 
oogonia  shown  in  Fig.  122,  C.  The  oogonium  is  dividing  and  forming  the 
female  gametes;  p,  paraphyses.  B,  the  female  gametes  discharged  but  still 
retained  in  the  inner  wall  of  the  oogonium.  C,  greatly  enlarged  view  of  a 
gamete  which  is  surrounded  by  male  gametes,  some  of  which  are  seen  as 
dark  bodies  penetrating  the  cytoplasm  of  the  female  gamete.  D,  early  stage 
in  the  germination  of  the  gametospore.  See  later  stage,  Fig.  121,  C. — After 
Thuret. 

one  after  fertilization  is  effected,  for  Farmer  observed  that  the 
male  cells  swarm  about  the  female  cells  for  a  time  and  then  sud- 
denly swim  away  "like  a  flock  of  frightened  birds."  As  soon  as 
fertilization  has  been  effected,  the  gametospores  become  invested 
with  a  cell  wall  and  attached  to  the  rocks.  Cell  division  now 
proceeds  rapidly  and  soon  establishes  the  characteristic  thallus 
of  the  plant  (Figs.  124,  D\  121,  C).  Fucus  is  a  very  prolific  plant 


210  THE   RED   ALGAE 

and  the  reproduction  process  can  be  observed  at  almost  any  time. 
After  the  plants  have  been  exposed  for  several  hours  by  the  low 
tide  the  male  and  female  gametes  will  often  be  found  forming 
orange  yellow  and  olive  green  drops  at  the  mouth  of  the  cavities. 
In  this  condition  the  entire  process  of  fecundation  and  early  stages 
of  germination  can  be  studied  by  transferring  a  bit  of  these  two 
fluids  to  a  drop  of  sea  water  on  a  slide  and  studying  them  under  a 
microscope.  The  differentiation  of  the  gametes  of  Fucus  is  an 
interesting  one  because  it  represents  the  stage  where  the  female 
gamete  has  become  motionless,  but  is  not  retained  in  the  mother 
cell  as  in  Vaucheria  and  Oedogonium. 

No  resting  spores  are  found  among  the  brown  algae.  This 
may  be  connected  with  the  more  uniform  conditions  that  ob- 
tain in  the  sea  where  also  they  are  not  exposed  to  the  dangers 
of  desiccation  as  in  fresh  water  forms.  It  is  noteworthy,  that 
although  the  reproductive  organs  are,  on  the  whole,  more  com- 
plex, fertilization  is  of  a  more  primitive  character  than  among 
many  of  the  green  algae,  in  that  the  fusion  of  the  gametes  is 
effected  outside  of  the  gametangia. 

CLASS  C.  RED  ALGAE  OR  RHODOPHYCEAE 
74.  General  Features. — The  Red  Algae  are  largely  marine 
plants.  Unlike  the  brown  algae,  they  reach  their  greatest  de- 
velopment and  abundance  in  the  warmer  waters  of  the  temperate 
and  tropical  seas  and  are  usually  found  attached  to  various  ob- 
jects below  tidal  marks.  Their  red  pigments  probably  adapt 
them  to  the  feeble  illumination  of  the  deep  waters  in  which  they 
generally  occur.  They  range  through  a  great  variety  of  forms, 
from  delicate  filaments  or  flattened  ribbon-like  bodies  to  struc- 
tures with  cylindrical  axes  and  leaf -like  branches  (Fig.  125). 
The  elegant  symmetry  of  their  branching  together  with  the  deli- 
cacy of  structure  and  richness  of  coloration  has  always  attracted 
attention  and  made  them  the  most  familiar  of  all  the  algae. 
They  are  popularly  though  inaccurately  known  as  sea  mosses. 

Reproduction  of  the  Red  Algae. — The  reproduction  of  the 
Rhodophyceae,  particularly  the  sexual  method,  presents  so  many 
modifications  and  specializations  that  only  the  general  features 


DEVELOPMENT   OF   PLANTS 


211 


can  be  pointed  out.  Singularly  both  the  spores  and  gametes 
have  lost  their  motility.  The  sexual  method  of  reproduction  is 
the  most  complicated  among  all  the  algae,  and  even  in  the 
simplest  form  is  far  in  advance  of  any  method  heretofore  noted  t 


FIG.  125.  Common  forms  of  the  Red  Algae:  A,  a  leaf-like  form,  Deles- 
seria.  B,  a  branching  thread-like  form,  Ceramium.  C,  a  branching  form 
covered  with  delicate  hairs,  Dasya. 

The  female  gametangium  resembles  a  flask  with  a  long  neck  and 
is  usually  developed  at  the  end  of  a  branch  (Fig.  126,  A),  see 
Coleochaete.  The  single  female  gamete  is  found  at  the  base  of 
the  flask.  The  male  gametes  are  produced  singly  in  small  cells 
also  at  the  ends  of  branches  and  they  are  often  closely  aggregated 
in  dense  clusters  (Figs.  126,  an;  127,  A,  B).  After  the  discharge 
of  the  male  gamete,  they  are  carried  by  the  currents  of  the 
water  to  the  long  tube  of  the  female  organ.  The  wall  of  the  tube 
at  the  point  of  contact  is  now  absorbed  and  the  male  gamete 
passes  down  the  tube  and  fuses  with  the  female.  The  gameto- 
spore  does  not  germinate  and  produce  a  new  plant  similar  to  the 
one  that  bore  the  gametes.  On  the  other  hand,  it  produces  a 
number  of  branches  (Fig.  126,  B,  C),  the  terminal  cells  of  which 
develop  as  spores  (Fig.  126,  D).  The  mode  of  sexual  repro- 


212     SEXUAL   REPRODUCTION   OF   RED   ALGAE 

duction  outlined  above  is  complicated  in  the  majority  of  the 
red  algae,  owing  to  the  fusion  of  the  germinating  gametospore 
with  adjacent  cells  that  contain  storage  foods  and  from  certain 
of  these  storage  cells  the  characteristic  spore  producing  branches 
are  developed.  In  this  way,  the  formation  of  a  larger  number 
of  spores  is  made  possible  by  a  single  fecundation.  Frequently 


O 


FIG.  126.  Sexual  reproduction  in  Nemalion:  A,  tip  of  branch  bearing 
female  gametangium,  c,  and  cluster  of  male  gametangia,  an,  from  some  of 
which  the  motionless  male  gametes  are  escaping.  B,  first  division  of  the 
germinating  gametospore,  g.  C,  later  stage  showing  the  early  formation 
of  the  branches  from  which  the  spores  will  be  developed.  D,  spores  forming 
at  the  ends  of  the  numerous  branches. 

a  sac-like  structure,  the  cystocarp,  is  developed  about  the  spores 
owing  to  the  outgrowths  of  the  adjacent  cells  (Fig.  127,  C).  Atten- 
tion should  be  fixed  upon  the  form'of  the  female  gametangium  and 
the  sac-like  structure  of  the  cystocarp  because  you  will  see  these 
features  reappear  in  certain  groups  of  the  fungi. 

The  spores  formed  from  the  growth  of  the  gametospore  are 
finally  set  free  and  grow  into  plants  resembling  the  gamete 
bearing  red  algae  but  singularly  these  plants  only  produce 
spores — never  gametangia.  These  spores  are  formed  in  fours 
in  a  single  cell  (Fig.  128)  and  therefore  called  tetraspores.  These 
spore-forming  cells  arise  singly  on  the  surface  or  in  the  tissues 


DEVELOPMENT   OF   PLANTS 


213 


FIG.  127.  Features  in  the  reproduction  of  Polysiphonia:  A,  branch  of 
the  plant  bearing  clusters  of  male  gametangia,  an.  B,  one  of  the  clusters 
enlarged,  showing  the  numerous  small  gametangia.  C,  branch  on  which  the 
spores  derived  from  the  gametospores  are  enveloped  by  a  sac.  From  one 
of  these  bodies,  called  cystocarps,  the  spores  are  escaping. 

of  the  plant,  or  they  may  be  associated  in  conspicuous  groups. 
The  spores  escape  from  the  mother  cell  as  naked  bodies  and 


FIG.    128.     A   branching    filament    of    Callithamnion,  showing   the  spore 
mother  cells  forming  tetraspores. 


214  LIFE   HISTORY  OF   ALGAE 

finally  secrete  cell  walls  and  develop  into  new  plants.  These 
plants,  however,  normally  bear  only  gametangia  and  not  tetra- 
spores.  So  in  this  life  history  we  have  two  similar  plants  appear- 
ing but  they  are  manifestly  different  in  nature  as  one  produces 
tetraspores  and  the  other  gametangia. 

The  red  algae  are  of  considerable  economic  importance. 
Irish  moss,  Chondrus,  is  used  in  the  manufacture  of  jellies,  and 
agar-agar  is  obtained  from  several  species  of  algae.  Many  tons 
of  various  kinds  of  red  algae  are  annually  dried  and  consumed 
for  food  in  the  East.  The  swallow's  nest,  of  which  you  have 
heard  so  much  as  an  article  of  food  in  the  Orient,  is  constructed 
of  algae. 

75.  Significant  Features  in  Life  History  of  the  Algae. — The 
germination  of  the  gametospore  in  the  Rhodophyceae  is  a  note- 
worthy departure  in  two  respects.  Unlike  previous  cases,  it  is 
retained  on  the  plant,  where  it  is  nourished  during  its  germination 
and  grows  practically  as  a  parasite.  This  relation  of  the  gameto- 
spore to  the  mother  plant  will  become  more  noticeable  among 
the  fungi  and  the  mosses  and  lead  to  pronounced  changes  in  the 
life  history  of  the  plant.  In  the  second  place,  we  notice  that  the 
gametospore  in  the  red  algae  produces  a  number  of  cells  before 
the  spores  are  formed.  In  the  green  and  brown  algae  it  either 
developed  directly  into  a  plant  similar  to  the  mother  plant,  as  in 
Vaucheria  and  Fucus,  or  zoospores  were  first  formed  which  de- 
veloped into  a  plant  similar  to  the  parent,  as  in  Ulothrix  and 
Oedogonium.  What  is  the  significance  of  this  variation  in  the 
germination  of  the  gametospore?  Attention  has  been  directed 
in  the  study  of  Spirogyra  and  Ulothrix,  p.  193,  to  the  fact  that 
there  are  two  phases  in  the  life  history  of  a  plant,  a  spore-bearing 
phase  or  generation,  and  a  sexual  phase  or  generation.  This  is 
doubtless  true  of  all  forms  characterized  by  a  sexual  reproduction. 
On  page  136  you  have  noticed  that  these  two  phases  or  genera- 
tions are  also  sharply  distinguishable  by  the  number  of  chromo- 
somes appearing  in  the  nuclei  of  the  cells,  the  sexual  generation 
having  only  half  the  number  found  in  the  asexual.  This  change 
in  the  number  of  chromosomes  occurs  at  two  points  in  the  life 
history,  first,  when  the  gametes  unite  to  form  the  gametospore, 


DEVELOPMENT   OF   PLANTS  215 

at  which  time  the  number  of  chromosomes  is  doubled,  and, 
secondly,  when  the  spore  mother  cell  of  the  asexual  generation 
divides  to  form  the  four  spores,  the  number  of  chromosomes  is 
reduced  by  one-half.  The  two  generations  are  generally  not 
apparent  in  the  Green  and  Brown  Algae,  because  the  asexual 
generation  is  greatly  reduced  and  often  does  not  produce  spores, 
as  in  Vaucheria,  etc.,  where  the  asexual  generation  consists  of  a 
single  cell,  the  gametospore.  This  gametospore,  however,  must 
have  twice  the  number  of  chromosomes  as  the  mother  plant,  since 
it  is  formed  by  the  union  of  the  chromatic  substance  of  two 
gametes  and  it  will  doubtless  be  found  that  its  first  divisions  in 
germination  are  attended  with  a  reduction  of  the  chromosomes 
to  the  original  number  found  in  the  mother  plant.  This  remark 
applies  also  to  Ulothrix  and  Oedogonium,  where  it  is  reasonable 
to  suppose  that  the  formation  of  the  four  zoospores  by  the  germi- 
nation of  the  gametospore  results  in  the  reduction  of  the  chromo- 
somes to  the  number  found  in  the  mother  plant.  This  has  been 
found  to  be  the  case  by  Allen  in  the  division  of  the  germinating 
gametospore  of  Coleochaete,  and  Yamanouchi  reports  that  the 
reduction  of  the  chromosomes  in  the  Red  Algae  does  not  occur 
until  the  mother  cell  divides  to  form  the  tetraspores  (Fig.  128) 
and  the  same  reduction  has  been  reported  in  Spirogyar  in  the 
germination  of  the  gametospore.  The  red  algae  give  us  the 
best  illustration  of  this  relationship  yet  seen.  Here  the  gameto- 
spore germinates,  producing  a  small  spore-bearing  plant,  enclosed 
in  the  cystocarp,  which  is  parasitic  upon  the  sexual  plant.  This 
plant  is  a  part  of  the  asexual  generation,  as  all  its  cells  contain 
the  double  number  of  chromosomes.  The  spores  from  the 
cystocarp  form  plants  externally  similar  to  the  sexual  plants, 
but  these  plants  are  really  different,  being  characterized  by  cells 
with  the  double  number  of  chromosomes  and  also  by  the  fact 
that  they  bear  tetraspores.  The  formation  of  the  tetraspores, 
however,  is  attended  with  a  reduction  of  the  chromosomes  by 
one-half,  and  these  spores  produce  plants  characterized  by  the 
reduced  number  of  chromosomes  and  also  by  the  fact  that  they 
bear  the  sexual  organs.  So  in  the  Red  Algae  we  have  very 
clearly  brought  before  us  the  two  generations  in  the  life  of  the 


2i6  THE  TRUE  FUNGI 

plant,  the  sexual  generation,  characterized  by  the  reduced 
number  of  chromosomes  and  the  production  of  the  sexual  organs, 
and  the  asexual  generation,  characterized  by  the  double  number 
of  chromosomes  which  appear  in  two  spore-bearing  plants,  i.  e., 
the  minute  plant  in  the  cystocarp  and  the  tetraspore-bearing 
plant.  The  distinction  between  the  sexual  and  asexual  genera- 
tion is  emphasized  at  this  point  because  we  see  in  the  algae  how 
it  gradually  became  more  and  more  conspicuous,  and  in  the 
mosses  and  succeeding  groups  it  will  be  noticed  that  the  relation 
of  these  two  generations  is  intimately  associated  with  the  evolu- 
tion of  the  higher  types  of  plant  life. 

Subdivision  5.  Eumycetes  or  True  Fungi 
76.  The  Nature  of  Fungi. — The  Fungi  are  the  largest  group 
of  the  Thallophyta  and  include  such  familiar  forms  as  moulds, 
mildews,  toadstools  and  mushrooms.  The  absence  of  chloro- 
phyll is  the  most  striking  feature  of  these  plants.  They  are 
unable  therefore  to  form  sugar,  starch  and  other  foods  from  the 
elements  of  the  soil  and  air  and  must  obtain  them  already  manu- 
factured. Consequently,  they  are  either  saprophytes  living  upon 
decaying  organic  matter,  or  parasites  preying  upon  living  organ- 
isms. It  is  quite  possible  that  this  mode  of  life  is  responsible 
for  the  disappearance  of  the  chlorophyll.  You  have  seen  that 
the  accumulation  of  sugar  in  the  chlorophyll  bearing  cells  stops 
their  activity.  So  here  the  absorption  of  organic  material  from 
plants  and  animals  may  have  had  the  same  result  and  so  led  to 
the  disappearance  of  chlorophyll.  The  majority  of  fungi  procure 
their  food  from  decaying  plant  and  animal  matter,  and  in  this 
relation  many  of  them  are  of  the  same  economic  importance  as 
the  saprophytic  bacteria.  Other  forms  exist  as  parasites  upon 
living  plants  and  animals.  In  this  relation  fungi  cause  almost 
incalculable  loss  annually  to  the  country  and  frequently  crops 
over  large  areas  are  ruined  by  their  depredations.  The  parasitic 
forms  gain  access  to  the  host  through  a  wound,  stoma  or  the 
delicate  tissues  of  seedlings  or  young  parts  of  the  plant,  and  in 
other  cases  the  fungus  forms  a  ferment  that  dissolves  the  cell 
walls  and  thus  opens  the  way  for  the  entrance  of  the  pest.  The 


DEVELOPMENT   OF   PLANTS  217 

cultivation  of  plants  has  doubtless  weakened  their  resistance  in 
many  cases  to  the  attack  of  the  parasites,  and  like  ourselves  the 
plant  may  inherit  a  weaker  constitution  that  is  more  subject  to 
disease.  Nearly  every  state  now  employs  experts  to  study  the 
diseases  caused  by  these  fungi  and  to  devise  means  for  killing 
them  and  to  develop  more  resistant  plants. 

The  fungi  are  of  very  simple  structure  because  they  live  upon 
foods  already  manufactured  for  them.  There  is  no  longer  a 
necessity  for  a  plant  body  that  is  complex,  because  each  part  is 
adapted  to  the  performance  of  one  or  another  of  the  many  func- 
tions that  cooperate  in  the  construction  and  distribution  of  the 
organic  substances.  Consequently,  the  fungi  do  not  exhibit 
many  of  the  characteristics  of  chlorophyll-bearing  plants.  Their 
cell  walls  are  thin  and  inclose  a  watery,  colorless  protoplasm  in 
which  are  usually  dispersed  many  small  nuclei  (Fig.  129).  What- 


FIG.  129.  A  few  cells  from  a  branching  filament  of  green  mould,  Peni- 
cillium,  showing  the  granular  character  of  the  cytoplasms  and  the  absence 
of  plastids.  The  colorless  areas  in  the  cells,  vacuoles,  contain  principally 
water. 

ever  form  the  plant  body  may  assume,  it  will  usually  be  found  to 
consist  of  filaments  of  delicate  cells  or  tubular  growths,  called 
hyphae  (sing,  hypha) .  These  fine  filaments  or  tubes  are  the  essen- 
tial portion  of  any  fungus  and  as  they  spread  over  the  substance 
upon  which  they  feed  they  form  a  branching  and  interwoven 
mass  of  threads  collectively  known  as  the  mycelium.  This  struc- 
ture is  well  illustrated  in  the  hyphae  that  spread  over  bread  and 
fruits,  forming  a  cobwebby  mass  or  mycelium  that  is  commonly 
known  as  mould  or  mildew.  Bodies  of  various  forms  arise  from 
the  mycelium  which  are  often  mistaken  for  the  fungus  itself,  as 
the  mushroom,  but  this  mushroom  sustains  the  same  relation  to 
15 


218  CLASSIFICATION   OF   FUNGI 

the  fungus  proper  (the  mycelium)  as  does  an  apple  to  the  apple 
tree.  The  mushroom  is  simply  a  rather  compactly  interwoven 
mass  of  hyphae  that  arise  from  a  mycelium  in  the  ground. 
We  shall  expect  to  find  remarkable  departures  in  the  fungi 
from  previous  types  because  of  their  peculiar  manner  of  ob- 
taining food  and  also  because  they  are  largely  terrestrial  plants 
and  therefore  exposed  to  a  much  wider  series  of  conditions  than 
in  the  case  of  the  algae.  This  will  tend  to  cause  variations  and 
so  bring  about  new  structures  that  are  in  harmony  with  the  con- 
ditions under  which  the  plants  live.  As  a  rule,  these  variations 
have  been  so  extensive  and  have  so  completely  changed  the  char- 
acter of  the  plant  as  to  render  it  impossible  to  state  whence  many 
of  the  groups  have  been  derived  or  what  relationship  they  sus- 
tain to  one  another.  It  may  be  stated,  however,  that  the  lower 
fungi  show  unmistakable  evidence  of  relationship  with  certain 
of  the  green  algae,  while  an  intermediate  class  have  evidently 
branched  off  from  the  red  algae.  In  nearly  all  cases  it  will  be 
seen  that  their  parasitic  and  saprophytic  mode  of  life  has  led  to 
reduction  aiid  degeneration.  The  fungi  may  be  divided  into 
three  classes:  A,  Phycomycetes  or  Alga-like  Fungi;  B,  Ascomy- 
cetes  or  Sac  Fungi;  C,  Basidiomycetes  or  Basidia-bearing  Fungi. 

CLASS  A.     PHYCOMYCETES 

77.  Alga-like  Fungi. — Some  of  these  plants  show  such  a  strik- 
ing resemblance  to  certain  algae,  both  in  the  structure  of  the 
plant  body  and  in  their  reproductive  processes,  that  they  are 
called  the  Phycomycetes  from  phycos,  alga,  and  myces,  mould. 
Attention  will  be  called  to  three  orders  of  this  group. 

78.  Order  a.     Saprolegniales  or  Water  Moulds. — These  fungi 
are  either  saprophytes,  living  upon  dead  animals  and  plants, 
or  parasites.     In  this  latter  relation  one  form  causes  a  destructive 
disease  in  fish  while  another  genus  produces  the  damping  off  and 
decay  of  seedlings.     They  are  all  microscopic  plants  and  many 
are  aquatic.     Common  examples  of  this  order  are  seen  in  the 
whitish  masses  that  form  around  decaying  insects  in  the  water 
and  in  fluffy  or  mould-like  outgrowths  that  often  appear  upon 
the  bodies  of  fish  in  aquaria.     The  plant  body  consists  of  branch- 


DEVELOPMENT   OF   PLANTS 


219 


ing  tubular  threads  without  partitions  but  containing  numerous 
nuclei  and  thus  resembling  Vaucheria  save  for  the  absence  of 
chloroplasts  (Fig.  130).  The  sporangia  are  also  formed  by  the 
cutting  off  of  the  tip  of  one  of  the  branches  by  a  transverse  wall. 
The  contents  of  a  sporangium,  however,  generally  breaks  up 
into  a  very  large  number  of  biciliate  zoospores  (Fig.  130,  C). 
In  the  species  that  cause  so  much  damage  to  fish,  the  spores 
come  to  rest  upon  the  fish  and  form  tubular  outgrowths  that 
readily  penetrate  the  tissues  of  the  fish,  especially  where  a  scale 
has  been  rubbed  off.  Some  of  the  hyphae  also  extend  outward 


FIG.  130.  Features  in  the  life  history  of  the  water  moulds:  A,  fly  in- 
fected with  Saprolegnia,  showing  the  tubular  threads  of  the  fungus  radi- 
ating out  from  its  body.  B,  tip  of  a  tube  magnified,  showing  an  early  stage 
in  the  formation  of  the  sporangium.  C,  sporangium  discharging  zoospores. 
D,  Reproductive  organs  of  a  related  fungus,  Achlya — o,  oogonium  contain- 
ing four  female  gametes;  an,  antheridium  with  tubes  penetrating  oogonium 
for  transport  of  male  gametes. 

from  the  body,  causing  the  mould-like  blotches  on  the  infected 
fish,  and  from  the  tips  of  these  branches  the  sporangia  mentioned 
above  are  formed.  Most  of  the  species  of  Saprolegnia  also  form 
sexual  organs  as  in  Vaucheria,  from  one  to  several  gametes  being 
formed  in  an  oogonium  (Fig.  130,  D).  The  antheridia,  however, 
develop  several  non-motile  male  gametes.  This  peculiarity  of 


220        REPRODUCTION   OF  A  WATER  MOULD 

the  gametes  is  perhaps  due  to  their  exposure  to  atmospheric 
conditions,  as  would  be  the  case  when  growing  upon  terrestrial 
or  floating  organic  matter.  The  lack  of  water  for  the  transport 
of  the  male  gametes  is  nicely  met  by  a  tubular  outgrowth  of 
the  antheridium  which  penetrates  the  oogonium  when  it  ruptures, 
discharging  the  male  close  to  the  female  gametes  (Fig.  130,  D, 
an) .  In  the  majority  of  the  species,  singularly  enough,  the  female 
gametes  germinate  without  being  fertilized.  The  gametospore 
germinates  as  in  Vaucheria. 

It  should  be  stated  that,  in  related  forms,  large  multiciliate 
zoospores,  like  those  of  Vaucheria,  are  formed  and  also  motile 
male  gametes.  It  is  also  important  to  note  that  there  are  several 
parasitic  algae  closely  related  to  these  alga-like  fungi  and  this 
remark  applies  to  the  Sac  Fungi  as  well.  Their  parasitic  habit 
and  partial  loss  of  chlorophyll  clearly  indicate  that  they  are  in  a 
transition  state  from  the  algae  to  the  fungi.  It  is  therefore 
safe  to  state  that  in  this  group  we  have  evidence  of  the  derivation 


FIG.  131.     Hyphae  of  Peronospora  extending  through  the  tissues  of  a  plant 
and  absorbing  food  from  the  cells  by  means  of  haustoria,  h. 

of  the  fungi  from  the  algae.  This  helps  us  to  understand  how 
fungi  arose.  Plants  and  animals  live  upon  organic  materials. 
These  substances  through  chemical  and  physical  reactions  cause 
the  absorbing  organs  of  the  plant  to  grow  towards  them.  Conse- 
quently the  green  plant  though  capable  of  forming  organic  sub- 
stances may  through  chemical  stimulation  be  brought  into  con- 
tact with  these  materials  in  other  plants  and  animals  and  as  a 
result  it  becomes  a  fungus. 

79.  Order  b.    Peronosporales.    Downy  Mildews  and  White 
Rusts. — These  forms  are  parasites  upon  the  higher  plants  espe- 


DEVELOPMENT   OF   PLANTS  221 

cially  and  include  two  of  the  most  destructive  fungi,  the  potato 
and  the  grape  vine  blight.  Phytophthora  infestans  causes  the 
potato  rot.  This  disease  was  widespread  in  the  eastern  United 
States  in  1901,  causing  the  entire  loss  of  the  crop  in  some  sections. 
It  would  be  difficult  to  imagine  a  pest  more  perfectly  adapted  to  a 
destructive  career.  Let  us  begin  the  life  history  with  the  germi- 
nation of  a  spore  which  has  fallen  upon  a  leaf.  This  spore  forms 
a  hypha  which  enters  the  leaf  and  quickly  spreads  through  its 
tissues,  sending  into  the  cells  short  lateral  branches  called  haus- 
toria  (Fig.  131),  which  absorb  the  cell  contents  and  thus  supply 
th3  fungus  with  food.  In  severe  cases  this  produces  a  withering 
and  decay  of  the  leaf.  The  hyphae  continue  their  growth  into 
the  stems  and  all  parts  of  the  plant,  thus  causing  the  black  dis- 
coloration of  the  potato  and  its  early  decay  in  bad  cases  of  in- 
fection. This  habit  of  many  parasitic  fungi  of  establishing  them- 
selves in  those  organs  of  the  plant  which  live  on  from  year  to 
year,  is  one  of  the  most  serious  difficulties  in  combating  these 
pests.  For  with  the  renewal  of  growth  of  these  organs  the  hyphae 
spread  and  reestablish  the  disease.  Thus  the  planting  of  infected 
potatoes  is  sure  to  result  in  the  appearance  of  the  potato  blight  if 
conditions  are  favorable.  As  soon  as  the  mycelium  has  become 
well  established  in  the  leaf,  numerous  branching  hyphae  extend 
out  through  the  stomata  and  form  at  their  tips  little  sacs  or 
sporangia  (Fig.  132,  A).  The  formation  of  the  sporangia  is 
effected  in  a  few  hours,  when  they  drop  off  and  are  carried  by  the 
wind  to  other  plants,  where  they  germinate  at  once,  forming  a 
tube  that  penetrates  the  leaf  and  rapidly  spreads  the  disease. 
If  the  sporangia  chance  to  fall  upon  leaves  that  are  wet  by  dew 
or  rain,  the  contents  breaks  up  into  several  zoospores  (Fig.  132, 
B-E)  which  finally  come  to  rest  and  develop  the  characteristic 
tubular  hyphae  of  the  fungi.  This  behavior  of  the  sporangium  is 
doubtless  a  survival  of  the  zoospore  stage  seen  in  the  algae.  It 
is  equally  suggestive  that  definite  changes  in  the  environment, 
as  the  dry  air,  causes  the  sporangium  to  germinate  as  a  non- 
motile  spore,  whereas  the  presence  of  water  causes  the  sporangium 
to  produce  several  zoospores.  The  potato  blight  is  largely 
confined  in  our  country  to  the  northeastern  states  and  usually  it 


222    REPRODUCTION  OF  THE  DOWNY  MILDEWS 

does  not  appear  until  the  latter  part  of  July.  It  only  becomes  of 
serious  importance  in  sultry  weather,  when  a  short  period  of 
rain  will  result  in  the  formation  of  the  sporangia  and  whole 
fields  will  be  devastated  in  a  few  days. 


FIG.  132.  FIG.  133. 

FIG.  132.  Asexual  reproduction  of  the  mildew:  A,  hyphae  of  Plasmo- 
para  emerging  from  a  stoma  and  bearing  numerous  sporangia.  B,  enlarged 
view  of  sporangium  of  Peronospora  germinating  on  a  dry  leaf.  In  this  case 
the  sporangium  behaves  as  a  spore  sending  out  a  hypha  that  will  penetrate 
the  tissues  of  the  leaf.  C,  sporangium  of  Phytophthora  germinating  in  the 
water  and  forming  zoospores.  D,  zoospore  enlarged.  E,  zoospore  has  come 
to  rest  and  is  forming  a  tube  that  will  penetrate  the  tissues  of  the  leaf  as 
in  the  case  of  B. 

FIG.  133.  Reproduction  of  the  white  rust,  Albugo:  A,  asexual  stage, 
showing  several  erect  hyphae  forming  spores.  B,  sexual  stage  which  is 
characteristic  of  the  Peronosporales  in  general — o,  gametangium  contain- 
ing a  single  female  gamete  which  is  being  penetrated  by  tube  from  male 
gametangium,  an. — After  Wager. 

Very  similar  in  character  is  the  grape  blight,  Plasmopara  viti- 
cola.  This  disease  causes  a  browning  of  the  leaves  and  the  stunt- 
ing of  the  fruit,  which  become  brown  or  gray,  and,  finally,  the 


DEVELOPMENT   OF   PLANTS  223 

death  of  the  infected  parts.  This  pest  caused  the  abandonment 
of  entire  vineyards  before  the  method  of  killing  the  fungus  by 
spraying  the  plants  with  copper  salts  was  discovered.  In  both 
of  these  pests  and  in  allied  genera  the  sporangia-bearing  hyphae 
are  produced  in  such  numbers  as  to  cause  a  downy,  mould-like 
appearance  on  the  leaves,  thus  accounting  for  their  popular 
name,  Downy  Mildews. 

In  a  related  genus,  Albugo,  the  sporangia  are  formed  by  the 
repeated  cutting  off  of  the  tips  of  the  hyphae,  as  shown  in  Fig. 
133,  A.  In  this  way,  not  one,  but  a  chain  of  sporangia,  are 
formed  from  the  ends  of  the  hyphae  which  do  not  project  from 
the  leaf,  but  grow  up  in  dense  masses  just  under  the  epidermis, 
producing  glistening  white  blotches  or  blisters  on  the  leaves. 
This  growth  finally  ruptures  the  epidermis  when  the  spores  are 
scattered  by  the  wind  and  germinate  as  in  the  preceding  cases. 
This  fungus,  known  as  white  rust,  is  very  common  on  mustards, 
pigweed  and  other  plants. 

The  sexual  reproduction  of  the  Peronosporales  is  suggestive  of 
Vaucheria.  Gametangia  are  cut  off  from  the  ends  of  the  hyphae, 
as  shown  in  Fig.  133,  B.  The  male  gamete  gains  access  to  the 
female  gamete,  which  is  usually  formed  singly,  by  means  of  a  tub- 
as in  Saprolegnia  (Fig.  13.3,  B,  an).  The  thick-walled  gametoe 
spore,  as  in  many  of  the  algae,  tides  the  plant  over  the  winter,  and 
being  set  free  by  the  decay  of  the  surrounding  tissues  it  germi- 
nates in  the  spring,  starting  anew  the  life  of  the  pest.  It  may 
germinate  directly  (see  Vaucheria)  into  the  fungus,  or  zoospores 
are  first  produced,  as  in  Oedogonium. 

These  two  orders  are  more  suggestive  of  the  algae  than  any 
others  that  we  shall  study  and  it  is  well  to  note  the  modifications 
that  have  been  induced  in  these  plants  as  a  result  of  their  change 
from  aquatic  to  terrestrial  conditions.  Removed  from  the  water, 
special  root-like  organs  and  haustoria  are  evolved  for  the  absorp- 
tion of  foods.  The  absence  of  water  brings  about  a  lack  of  motil- 
ity  in  the  male  gametes  and  the  formation  of  a  tube  to  conduct 
them  to  the  female.  For  the  same  reason,  the  zoospores  are  re- 
duced to  light  motionless  spores  that  are  developed  upon  elon- 
gated hyphae  that  expose  them  to  the  air  currents  for  distribution. 


224          REPRODUCTION  OF  BLACK   MOULDS 

The  ability  of  these  spores,  or  sporangia,  to  produce  zoospores  is 
doubtless  the  survival  of  a  trait  inherited  from  their  algal  an- 
cestors. 

80.  Order  c.  Mucorales  or  Black  Moulds. — These  are  among 
the  most  common  of  the  fungi  and  they  are  almost  sure  to  appear 
upon  any  cooked  food  or  decaying  matter  that  is  exposed  to  the 
air  even  for  a  very  short  time  (Fig.  134).  The  mycelium  has 
practically  the  same  structure  as  noted  in  the  preceding  groups, 
but  the  black  moulds  have  lost  all  motile  reproductive  bodies  and 
their  relationship  to  any  group  of  the  algae  is  not  known.  Some 
of  the  hyphae  of  the  mycelium  creep  over  the  food  supply  and 
send  into  it  short  branches  which  serve  as  organs  of  absorption 
while  other  rather  thicker  hyphae  grow  away  from  the  mycelium 
and  reach  up  into  the  air  (Fig.  134).  The  tips  of  these  erect 


FIG.   134.     Habit  of  growth  of  the  black  mould,  Rhizopus:  s,  sporangia; 
r,  absorbing  branches  of  the  mycelium. 

hyphae  enlarge  owing  to  the  accumulation  of  protoplasm  and 
numerous  nuclei  in  them,  and  finally  become  spherical,  forming 
the  sporangia.  The  contents  of  the  young  sporangia  differentiate 
into  a  denser  peripheral  portion  containing  the  bulk  of  the  nuclei 
and  a  more  watery  central  and  basal  region.  A  layer  of  large, 
round  vacuoles  now  appears  between  these  two  regions  and, 
owing  to  the  flattening  out  and  fusion  of  these  vacuoles,  a  cleft 
is  formed  that  ultimately  separates  the  peripheral  from  the  central 
protoplasm.  Next  furrows  advancing  inward  from  the  sporangial 
wall  and  outward  from  the  cleft  divide  the  peripheral  protoplasm 
into  numerous  small  portions,  each  part  containing  several  nuclei. 
These  bodies  round  off,  secrete  a  rather  thick  smoky  black  wall 


DEVELOPMENT   OF   PLANTS 


225 


and  so  become  spores  (Fig.  135).  The  numerous  sporangia  filled 
with  dark  spores  are  the  principal  cause  of  the  black  color  of  these 
fungi. 

During  the  development  of  the  spores  a  wall  is  constructed 
over  the  central  protoplasm,  thus  often  forming  a  dome-like 
structure  in  the  sporangium  known  as  the  columella  (Fig.  135,  jE). 
The  walls  of  the  sporangia  (save  at  the  region  in  contact  with 
the  stalk)  readily  dissolve  in  the  presence  of  moisture,  owing 
to  their  mucilaginous  character,  and  thus  allow  the  spores  to 
float  off  in  the  air  as  an  invisible  dust.  The  spores  will  germi- 
nate at  once  and  produce  a  new  plant  under  suitable  conditions 
of  moisture  and  food,  or  they  will  retain  their  vitality  for  months 
if  kept  dry.  As  the  spores  are  disseminated,  the  dome-like  struc- 


FIG.  135.  A,  B,  early  stages  in  the  development  of  the  sporangium  of 
Rhizopus.  C,  the  formation  of  the  vacuoles  which  separate  the  denser  peri- 
pheral protoplasm  from  the  more  watery  central  region.  D,  the  vacuoles 
are  flattening  out  and  the  denser  protoplasm  is  becoming  separated  into 
multinucleate  segment,  E,  the  multinucleate  segments  are  rounding  off  to 
form  the  spores  and  a  wall  has  formed  over  the  columella.  F,  the  sporan- 
gium has  ruptured,  permitting  the  scattering  of  the  spores  and  the  columella 
has  formed  an  umbrella-like  structure  owing  to  the  loss  of  its  watery  con- 
tents.— After  Swingle. 

ture  in  the  sporangium  relaxes,  owing  to  the  loss  of  its  watery 
contents,  and  assumes  an  umbrella  shape,  as  seen  in  Fig.  135,  F. 
You  can  readily  understand  why  these  black  moulds  are  so  com- 
mon by  counting  the  number  of  sporangia  on  a  small  bit  of  myce- 
lium and  then  estimating  the  number  of  spores  in  a  sporangium. 
So  numerous  and  light  are  the  spores  that  they  are  carried  every- 
where. It  is  only  necessary  to  expose  a  bit  of  moist  bread  for  a 
few  moments  to  the  air  and  then  enclose  it  in  a  damp  chamber  to 
secure  a  luxuriant  crop  of  these  plants.  An  interesting  variation 


226 


SPORANGIUM    OF   PHILOBOLUS 


in  this  mode  of  scattering  the  spores  is  seen  in  a  related  form, 
Pilobolus,  which  is  of  common  occurrence  upon  horse  dung. 
Here  the  wall  of  the  sporangium,  unlike  Rhizopus,  is  quite  firm 
and  becomes  mucilaginous  only  at  the  point  of  contact  with  the 
stalk  that  bears  it.  Owing  to  the  accumulation  of  water  in  the 
stalk  such  a  pressure  is  finally  set  up  as  to  rupture  it  at  the 
mucilaginous  point  and  so  the  sporangium  with  its  contained 
spores  is  hurled  considerable  distance — often  quite  one  meter. 
The  sporangia  bearing  stalks  are  also  sensitive  to  light.  If 
a  glass  jar  covered  with  black  paper  so  as  to  exclude  all  light  is 


FIG.  136.  Sexual  reproduction  in  the  black  mould:  A,  the  meeting  of 
the  tips  of  two  hyphae.  B,  later  stage,  the  lower  part  of  the  figure  shows 
the  cutting  off  of  the  tips  by  transverse  walls  and  in  the  upper  part  of  the 
figure  the  fusion  of  the  contents  of  the  two  gametangia  thus  formed  has  be- 
gun. C,  mature  gametospore. 

placed  over  a  colony  of  these  plants  and  a  minute  opening  is  now 
made  in  the  paper,  you  will  be  surprised  to  note  with  what 
accuracy  each  sporangium  is  shot  off  and  hits  this  opening. 
The  results  are  the  same  no  matter  where  you  make  the  opening. 
Under  certain  conditions  the  sexual  method  of  reproduction  is 
effected  by  the  union  of  two  club-shaped  hyphae,  as  shown  in 
Fig.  136,  A.  As  these  hyphae  meet,  the  tip  of  each  branch  is 
cut  off  by  a  wall  and  the  contents  of  the  two  tips  fuse,  forming  a 


DEVELOPMENT   OF   PLANTS 


227 


thick-walled  gametospore  (Fig.  136,  B,  Q.  After  a  resting 
period,  this  gametospore  grows  directly  into  a  new  mycelium. 
In  some  of  the  black  moulds  Blakeslee  has  made  known  that 
reproduction  is  only  effected  by  the  union  of  hyphae  from 
different  plants  which  must  differ  therefore  in  their  nature,  just 
as  you  found  to  be  the  case  in  some  species  of  Spirogyra. 

81.  A  Fly  Fungus. — An  interesting  form  allied  to  the  mucors 
is  seen  in  the  parasitic  fungus  Empusa,  that  produces  an  epi- 


B 


FIG.  137.  A  fungus,  Empusa,  parasitic  upon  flies:  A,  fly  surrounded  by 
a  mass  of  discharged  sporangia.  B,  enlarged  view  of  several  hyphae,  showing 
the  discharge  of  the  sporangia  which  are  surrounded  by  a  mucilaginous  sub- 
stance.— After  Brefeld. 

demic  among  flies  at  certain  seasons  when  they  may  be  seen  dead 
and  clinging  to  the  woodwork  and  window  panes  surrounded  by 
a  white  halo  (Fig.  137,  A).  This  appearance  is  caused  by  the 
discharge  of  numerous  white  sporangia  that  are  found  at  the 
ends  of  the  hyphae  that  project  from  all  sides  of  the  fly's  body 
(Fig.  137,  B).  If,  by  chance,  a  fly  should  come  within  range 


228  THE   SAC   FUNGI 

of  these  discharging  sporangia,  it  would  become '  infested  and 
later,  branches  of  the  hyphae  would  project  from  its  body  and 
repeat  the  process  of  sporangia  formation. 

CLASS  B.  ASCOMYCETES  OR  SAC  FUNGI 
82.  General  Characters. — The  Ascomycetes  are  the  largest  and 
most  variable  group  of  the  fungi.  They  are  illustrated  by  the 
powdery  mildews  which  affect  the  leaves  of  a  variety  of  plants; 
the  brown  and  blue  moulds  that  occur  on  preserves,  decaying 
fruit,  old  leather,  etc.  (Fig.  138);  the  cup  fungi  and  morels 
(Figs.  139,  141);  the  black  knot  of  the  cherry  and  plum  trees 
(Fig.  149),  and  the  ergot  of  rye,  etc. 

The  plant  body  or  mycelium  resembles  in  structure  that  of 
preceding  orders,  but  its  hyphae  are  composed  of  numerous  cells 
instead  of  being  tubular  or  with  few  cross  walls  as  in  the  Phyco- 
mycetes  (Fig.  138).  The  sporangia  (Fig.  152)  are  reduced  in 


FIG.  138.  The  green  mould,  Penicillium,  one  of  the  most  common  of  the 
Sac  Fungi.  The  hyphae  of  the  branching  mycelium  is  composed  of  cells 
and  the  spores  or  conidia  are  formed  in  chains  that  are  arranged  in  brush- 
like  clusters  at  the  ends  of  the  erect  hyphae. 

size  and  contain  but  a  single  spore  each,  which  is  never  dis- 
charged from  the  sporangium.  This  body  will  be  referred  to 
in  the  future  as  a  spore  or  conidium  (plu.  conidia). 

The  gametangia  are  suggestive  of  those  noted  in  the  red  algae 
(Fig.  140),  though  in  many  forms  they  appear  to  be  greatly  re- 
duced or  even  lacking.  The  fusion  of  the  gametes  results  in  the 
formation  of  a  gametospore  which  germinates  at  once,  forming  in 


DEVELOPMENT   OF   PLANTS  229 

the  simplest  cases  one  or  more  sacs.  These  bodies  are  known  as 
asci  (sing,  ascus)  and  they  contain  more  frequently  eight  spores, 
called  ascospores  (Fig.  160),  which  must  be  distinguished  from 
the  spores  or  conidia  mentioned  above.  The  gametospore  germi- 
nates as  a  parasite  on  the  mother  plant  as  in  the  red  algae. 
The  sexual  process  results,  in  the  majority  of  forms,  not  only 
in  the  formation  of  asci,  but  numerous  hyphae  from  the  my- 
celium are  also  stimulated  to  growth  and  become  associated 
with  the  asci  in  various  ways,  recalling  the  cystocarps  of  the  red 
algae.  As  a  result  fruit  bodies  of  various  forms,  called  ascocarps 
or  perithecia,  are  developed  which  often  become  the  conspicuous 
part  of  the  fungus  (Figs.  155-158).  The  ascospores  germinate 
immediately  or  after  a  period  of  rest  and  form  a  new  mycelium. 
It  should  be  stated  that  the  ascocarps  have  not  been  connected, 
in  many  cases,  with  sexual  processes,  and  it  is  inferred  that  sexu- 
ality has  been  lost  along  with  other  characters  as  a  consequence 
of  the  degeneracy  resulting  from  parasitic  and  saprophytic 
habits.  Only  a  few  of  the  more  important  orders  will  be  dis- 
cussed. 

83.  Order  a.     Pezizales  or  the  Cup  Fungi. — These  plants  are 
characterized  by  the  formation  of  fleshy,  leathery  or  gelatinous 


FIG.  139.     One  of  the  common  cup  fungi,  order  Pezizales,  with  broadly 
opened  ascocarps.     Common  upon  rich  humus  soil  and  decaying  wood. 

cup-like  ascocarps  that  range  in  size  from  mere  specks  to  forms 
four  or  five  inches  in  diameter  (Fig.  139).  The  mycelium  lives 
upon  the  humus  in  the  ground  or  on  decaying  plants  and  ap- 
parently in  this  and  the  next  order  frequently  develops  the  asco- 
carps directly  without  a  reproductive  process.  In  Pyronema 
and  many  other  forms,  however,  Dodge  has  shown  that  reproduc- 


230 


REPRODUCTION   OF   THE   PEZIZALES 


tive  organs  are  formed  that  are  very  suggestive  of  those  of  the 
red  algae.  The  female  gametangium  in  the  simpler  forms  has 
an  enlarged  basal  region  with  a  long  tubular  outgrowth  sug- 
gestive of  Nemalion  (Fig.  140).  In  the  majority  of  forms  this 
organ  consists  of  several  cells  and  often  becomes  coiled  and 
greatly  extended.  The  male  gametangium  is  a  somewhat 
enlarged  hypha.  These  organs  are  developed  here  and  there  on 
the  mycelium  or  several  may  be  associated  in  a  compact  group. 


B 


FIG.  140.  Sexual  reproduction  of  one  of  the  cup  fungi,  Pyronema:  A, 
the  flask-shaped  female  gametangium,  o,  is  seen  in  various  stages  of  fusion 
with  the  male  gametangium,  an.  This  is  effected  by  the  curvature  of  the  tu- 
bular outgrowth.  B,  the  gametospores,  g,  germinating  and  forming  branching 
hyphae  which  bear  the  asci,  as.  These  asci  are  associated  with  hyphae  or 
paraphyses,  p,  that  arise  from  the  mycelium.  The  asci  and  paraphyses 
constitute  the  hymenium. — After  Harper. 

Singularly  the  union  between  these  two  sex  organs  is  effected 
by  the  female — its  tubular  part  curves  towards  the  male  game- 
tangium and  effects  a  fusion  with  it.  As  to  whether  the  male 
gametes  pass  over  and  unite  with  the  females  as  a  result  of  the 
fusion  of  the  gametangia  no  general  statement  can  be  made 
at  present,  though  in  some  forms  there  is  evidence  to  indicate 
that  such  in  the  case.  The  basal  portion  of  the  female  game- 
tangium, or  one  of  its  cells  in  cases  where  the  gametangium  con- 
sists of  several  cells,  behaves  as  though  fertilization  had  been 


DEVELOPMENT   OF   PLANTS  231 

effected  and  a  gametospore  had  been  formed.  It  germinates  by 
developing  a  number  of  erect  sac-like  hyphae  or  asci  (Fig.  140,  J5). 
Adjoining  hyphae  grow  up  among  the  asci  and  around  them, 
forming  a  cup-like  structure  resembling  that  shown  in  Fig.  139. 
A  section  through  one  of  these  cups  reveals  the  asci  intermingled 
with  hyphae,  also  called  paraphyses,  in  the  form  of  a  layer  or 
stratum.  Such  an  association  of  spore-bearing  organs  and  para- 
physes is  called  a  hymenium.  The  spores  are  discharged  to  con- 
siderable distances,  owing  to  the  accumulation  of  fluids  in  the 
asci  which  finally  rupture  at  their  apices.  They  are  formed 
in  such  enormous  numbers  that  under  favorable  conditions  they 
may  be  seen  passing  off  as  faint  puffs  of  smoke.  The  bright 
scarlet  cups  of  Sarcoscypha,  common  in  the  spring  on  decaying 
sticks  and  the  gray  cups  of  Peziza  growing  upon  the  ground 
and  decaying  wood,  are  familiar  examples  of  the  order. 

84.  Relationship  of  the  Cup  Fungi. — The  Pezizales  may  be 
regarded   as  the   typical   representatives   of   the   Ascomycetes. 
The   sexual   reproductive   process  as  outlined   above   is   fairly 
representative  of  the  other  groups  of  ascomycetes  and  will  not 
be  referred  to  again  except  in  orders  where  noteworthy  departures 
occur.     The  formation  of  spores  or  conidia  are  characteristic 
features  of  many  of  the  groups  but  owing  to  the  exceedingly 
diverse  character  and  origin  of  these  bodies  attention  will  only 
be  directed  to  some  of  the  more  familiar  and  common  examples 
as  illustrated  in  the  two  last  orders.     Diverging  from  these  typi- 
cal ascomycetes  are  several  lines  of  fungi  that  show  varying  de- 
grees of  relationship  to  them.     As  an  example  of  this  divergence 
attention  may  first  be  directed  to  the  Helvellales. 

85.  Order  b.     Helvellales.— These  are  fleshy  forms  like  the 
cup  fungi  and  grow  upon  the  ground,  but  the  reproductive  process 
results  in  the  development  of  oddly  shaped  ascocarps  with  the 
hymenium  developed  upon  the  upper  exposed  surface.     Several 
common  forms  of  these  ascocarps  are  illustrated  in  Fig.  141. 

The  morel  or  Morchella,  in  which  the  hymenium  is  spread  over 
the  pitted  or  honey-combed  surface  of  the  ascocarp,  is  one  of 
the  most  highly  prized  of  the  edible  fungi.  The  members  of 
this  order  are  largely  saprophytic  and  often  attain  considerable 


232 


THE    ASCO-LICHENS 


size,  forms  of  Morchella  occasionally  reaching  the  height  of  a 
foot  and  some  species  of  Gyromitra  weigh  over  a  pound. 

86.  The  Asco-lichens. — A  second  line  of  departure  from  the 
Pezizales  includes  a  large  group  of  plants  known  as  the  lichen. 
The  great  majority  of  these  forms  show  strong  evidence  of  rela- 


FIG.  141.  Common  forms  of  the  Helvellales:  A,  the  morel,  Morchella, 
surface  view  at  left  and  in  section  at  right.  The  asci  and  paraphyses  form 
a  hymenium  over  the  honeycomb  surface.  B,  Leotia,  a  small  gelatinous 
form  of  a  light,  bluish-green  color.  C,  Geoglossum,  a  black  tongue-like  form. 
In  B  and  C  the  hymenium  is  confined  to  the  upper  enlarged  portion  of  the 
fungus.  Both  are  common  in  boggy  ground. 

tionship  with  the  cup  fungi  in  their  reproductive  processes  and 
it  should  be  added  that  the  sex  organs  are  more  suggestive  of 
the  red  algae  than  in  any  other  group.  A  few  species  belong  to 
the  third  class  of  fungi,  the  basidiomycetes,  but  these  plants  show 
the  same  general  features  as  the  asco-lichens  and  therefore  a 
consideration  of  the  entire  group  may  be  taken  up  at  this  point. 
These  remarkable  plants  are  of  almost  universal  distribution  upon 
tree  trunks,  rocks,  old  fences  and  buildings,  and  upon  the  bare 
earth,  where  they  form  variously  colored  incrustations  or  leaf- 
like  branching  bodies  (Fig.  142). 

The  lichen  is  one  of  the  most  extraordinary  plants  in  the 
vegetable  kingdom,  since  it  is  a  union  of  two  separate  plants,  a 
fungus  and  an  alga.  Naturally  the  relationship  of  the  lichens 
to  other  groups  of  plants  has  been  a  matter  of  dispute,  some 


DEVELOPMENT   OF   PLANTS 


233 


regarding  them  as  constituting  an  independent  division,  and  by 
others  they  are  looked  upon  as  fungi.  The  fact  that  the  fungus 
forms  the  bulk  of  the  lichen  and  lives  upon  the  alga  somewhat 
after  the  manner  of  a  parasite  and  is  usually  alone  capable  of 
forming  reproductive  spore  bodies  would  lead  to  the  latter 


FIG.  142.  Common  species  of  Lichens:  A,  an  erect  branching  form, 
Cladonia — as,  ascocarps.  B,  a  foliaceous  lichen,  Sticta.  C,  Parmelia  spread- 
ing over  the  bark  of  tree.  The  centrally-placed  ascocarps  are  surrounded 
by  smaller  pycnidia. 

position.  However,  all  degrees  of  relationship  appear  between 
the  alga  and  fungus — from  stages  where  their  association  is  of 
little  or  no  consequence  to  stages  where  their  existence  is  abso- 
lutely dependent  upon  their  association.  The  members  of  this 
curious  co-partnership  are  largely  ascomycetes  and  blue-green 
algae.  Numerous  species  in  each  of  these  two  groups  of  plants 
have  become  accustomed  to  living  together.  The  formation  of 
a  lichen  comes  about  through  the  attachment  of  a  hypha  of  some 
fungus  to  one  or  more  algal  cells,  as  shown  in  Fig.  143,  B.  In 
this  way,  the  food  manufactured  by  the  algae  is  absorbed  os- 
motically  by  the  hyphae.  In  some  cases,  the  fungus  obtains 
its  food  by  means  of  haustoria  which  penetrate  the  algal  cells. 
By  the  continued  growth  of  the  fungus  and  algae  there  finally 
results  an  interwoven  mass  of  hyphae  about  the  algae.  The 
walls  of  the  fungus  are  sufficiently  transparent  to  permit  the 
entrance  of  light  and  the  color  of  the  green  algae  can  readily 
be  seen  when  the  lichen  is  moistened. 

(a)  Structure  of  the  Lichen. — The  bulk  of  the  lichen  is  more 
usually  composed  of  hyphae  which  show  considerable  regularity 
16 


234  STRUCTURE   OF   LICHENS 

in  their  growth  and  the  majority  of  the  algae  also  are  usually 
confined  to  a  definite  zone  near  the  sunned  surface  of  the  lichen. 
Thus,  in  Fig.  143,  A<  which  represents  a  cross-section  of  a  lichen, 
it  will  be  seen  that  the  fungus  forms  a  rather  firm  layer  at  the 
top  and  bottom  of  the  thallus,  while  the  algae  are  distributed  near 
the  upper  surface,  where  they  are  exposed  to  the  light,  and  can 
therefore  carry  on  photosynthesis.  Numerous  hyphae  projecting 
from  the  under  surface  of  the  lichen  serve  to  anchor  it  to  the 
substratum  and  also  assist,  doubtless,  in  the  absorption  of  the 
earth  substances.  In  some  of  the  gelatinous  lichens  the  fungi 
and  algae  are  more  promiscuously  arranged. 

While  the  fungus  is  dependent  upon  the  foods  manufactured 
by  the  algae,  the  latter  are  also  benefited  to  an  extent  by  this 
arrangement  since  they  are  protected  by  the  strata  of  hyphae,  and 
they  are  also  provided  with  water  and  crude  material  which  the 
fungus  readily  takes  up  and  holds.  This  mutually  helpful  rela- 
tionship of  two  organisms  is  a  form  of  symbiosis,  termed  com- 
mensalism.  The  strange  feature  about  this  co-partnership  is  the 
marked  change  produced  in  the  nature  of  the  two  symbionts.  As 
long  as  they  are  independent  of  each  other,  very  special  conditions 
are  necessary  for  their  welfare,  but  associated,  they  form  the  most 
resistant  plants  known.  This  accounts  for  their  distribution  from 
the  equator  to  the  pole  and  their  association  upon  crystalline 
rocks,  baked  earth,  bark  of  trees  and  other  places  where  no  other 
plant  life  is  possible.  Under  unfavorable  conditions  the  lichen 
becomes  dry  and  brittle,  in  which  resting  condition  it  is  able  to 
meet  any  extreme  temperature  and  drought.  With  the  return  of 
suitable  moisture  and  heat  they  become  leathery  or  gelatinous 
and  renew  their  growth  with  considerable  rapidity.  Thus  they 
live  on  from  year  to  year,  but  owing  to  the  exposed  places  in  which 
they  are  usually  found,  their  growing  periods  are  frequently  very 
short  and  their  total  annual  growth  may  not  exceed  a  few  milli- 
meters— see  soils,  p.  50. 

(b)  Reproduction  of  Lichens. — Reproduction  of  the  lichens  is 
brought  about  by  means  of  fragments  that  are  readily  detached 
when  the  lichens  are  dry  and  brittle,  or  by  means  of  soredia. 
These  latter  bodies  consist  of  a  few  algal  cells  intertwined  with 


DEVELOPMENT   OF   PLANTS 


235 


hyphae,  in  fact,  a  miniature  lichen  (Fig.  143,  C).  In  some  species 
the  soredia  form  a  rather  powdery  or  granular  coating  on  the 
upper  surface  of  the  lichen,  and  in  other  cases  they  are  developed 
within  the  lichen.  These  bodies  are  easily  scattered  by  the 
wind  when  the  lichens  are  dry  and  under  favorable  conditions 
grow  into  new  lichens.  Reproduction  is  also  effected  by  means  of 
ascospores  that  are  developed  as  in  the  cup  fungi.  The  female 


B 


C1 


FIG.  143.  FIG.  144. 

FIG.  143.  Structure  of  the  Lichen:  A,  section  of  a  lichen,  showing  the 
compact  arrangement  of  the  hyphae  at  the  top  and  bottom,  also  the  anchor- 
ing fungal  threads  on  the  underside  and  the  dark  algal  cells,  a,  near  the  top. 
B,  enlarged  view  of  the  algae  to  show  their  relation  to  the  hyphae.  C,  dia- 
gram of  one  of  the  powdery  particles,  soredium,  appearing  upon  certain 
lichens,  showing  the  hyphae  and  algae.  These  bodies  are  scattered  by  the 
wind  and  form  new  lichens. 

FIG.  144..  Sexual  reproduction  of  the  lichen:  A,  section  of  an  ascocarp, 
the  hymenium  appearing  as  a  dark  band  in  the  mouth  of  the  cup.  B,  en- 
larged view  of  the  asci,  a,  and  paraphyses,  p,  of  the  hymenium. 

gametangium  is  a  coiled  organ  consisting  of  many  cells  as  in 
some  Pezizales  but  the  male  gametes  are  developed  in  ascocarp- 
like  bodies  where  they  are  formed  at  the  ends  of  numerous 
minute  hyphae — in  their  origin,  therefore,  being  strikingly  sug- 
gestive of  the  red  algae.  In  one  form,  it  should  be  added  that  a 
fusion  between  male  and  female  gametangia  has  been  reported. 


236  IMPORTANCE   OF   LICHENS 

The  ascocarps  derived  from  this  reproductive  process  assume  a 
great  variety  of  shapes  (Fig.  142)  but  they  all  show  the  hymenial 
layer  of  asci  and  paraphyses  (Fig.  144)  or  of  basidia  and  para- 
physes  in  the  few  forms  belonging  to  basidio-lichens. 

The  ascospores  germinate  readily,  producing  hyphae  which, 
however,  soon  perish  unless  they  chance  to  meet  an  alga  with 
which  they  can  live.  It  should  be  noted  that  these  fungi  appear- 
ing in  lichens  cannot  associate  indiscriminately  with  any  species 
of  algae.  Each  species  of  fungus  is  adapted  to  one  or  more  species 
of  algae  and  is  unable  to  live  with  any  other  form.  As  may  be 
imagined,  the  real  nature  of  the  lichen  was  for  a  long  time  mis- 
understood, the  algae  even  being  looked  upon  as  spore  bodies. 
The  bitter  dispute  over  the  question  was  finally  settled  when  a 
lichen  was  produced  artificially  by  bringing  together  a  suitable 
fungus  and  alga. 

(c)  Economic  Value. — Lichens  are  of  considerable  economic 
importance  in  the  world.  Their  service  in  hastening  the  decay 
and  transformation  of  rock  material  has  already  been  referred  to. 
In  many  sections  they  form  the  characteristic  vegetation  of  the 
country,  as  in  certain  alpine  and  desert  regions  and  in  the  north- 
ern barrens,  where  they  afford  rich  pasturage  to  reindeer  and 
caribou.  Their  abundant  growth  in  mountainous  districts  and 
subsequent  distribution  by  winds  and  rains  accounts  for  the 
showers  of  manna  in  biblical  history.  Certain  species  are  still 
used  in  northern  regions  to  lengthen  out  a  meager  food  supply. 
Litmus,  employed  in  acid  testing,  and  various  pigments  are 
derived  from  lichens,  as  were  also  the  famous  purple  and  blue 
dyes  of  the  East. 

87.  A  Third  Line  Possibly  Related  to  the  Pezizales.— The 
following  orders  may  not  represent  a  single  line  of  descent  but 
there  are  certain  features  in  their  life  history  indicating  that 
they  may  represent  a  line  of  departure  from  the  Pezizales. 
These  forms  are  mostly  minute;  parasitic,  or  saprophytic  upon 
dead  plants.  The  more  epiphytic  position  of  these  forms  may 
have  been  a  factor  in  the  reduction  in  size.  Certainly  the 
larger  fleshy  forms  of  the  preceding  orders  are  terrestrial  in 
habitat.  There  are  two  quite  distinct  groups  in  this  alliance, 


DEVELOPMENT   OF   PLANTS 


237 


the  one  characterized  by  ascocarps  that  are  only  slightly  opened 
so  that  the  %asci  are  not  as  freely  exposed  to  the  air  as  in  the 
preceding  orders  and  the  other  with  closed  ascocarps  so  that  the 
asci  are  exposed  only  on  the  decay  of  the  fruit  body.  The 


FIG.  145.  FIG.  146. 

FIG.  145.  Head  of  rye  infested  by  the  parasite,  Claviceps,  which  has 
transformed  several  of  the  grains  into  black  masses  of  mycelium  known  as 
sclerotia. 

FIG.  146.  Various  phases  in  the  life  history  of  Claviceps:  A,  a  young 
grain  or  pistil  infested  with  the  parasite.  B,  enlarged  view  of  the  mycelium 
as  it  appears  on  the  surface  of  the  pistil,  showing  the  formation  of  numerous 
spores.  C,  the  hyphae  of  a  sclerotium  growing  out  and  forming  several 
purplish  stalks,  each  capped  with  knob-like  clusters  of  ascocarps.  D,  en- 
larged sectional  view  of  one  of  these  knobs,  showing  numerous  ascocarps 
on  the  periphery.  E,  one  of  the  ascocarps  enlarged,  as,  asci.  F,  an  ascus 
containing  eight  thread-like  ascospores. 


238  LIFE   HISTORY   OF   CLAVICEPS 

first  group  is  further  characterized  by  the  fact  that  the  ascocarps 
are  frequently  associated  with  a  more  or  less  conspicuous  growth 
of  the  mycelium  in  which  the  ascocarps  are  imbedded  in  varying 
degrees.  This  growth  is  known  as  the  stroma  and  it  may  form 
a  rather  compact  mass  of  hyphae.  Only  two  orders  can  be  con- 
sidered in  each  of  these  two  groups. 

88.  Order  c.  Hypocreales. — These  fungi  are  distinguished  by 
their  rather  fleshy  or  membranous  ascocarps  and  stroma,  which 
range  in  color  from  white  to  yellow,  purple,  scarlet  and  brown 
Numerous  species  are  saprophytic  while  others  are  parasitic 
upon  higher  plants,  fungi  and  insects.  Clamceps  is  a  common 
example  of  this  group,  causing  the  disease  known  as  ergot  in  the 
flowers  of  rye  and  other  grasses  (Fig.  145).  The  mycelium  at 
first  spreads  over  the  outer  part  of  the  pistil,  rapidly  forming 
spores  (Fig.  146,  A,  B)  and  exuding  a  sweet  slimy  juice,  honey 
dew,  which  is  eagerly  eaten  by  flies.  In  this  way  the  spores  are 
carried  away  to  infest  other  plants.  The  mycelium  finally 
completely  absorbs  the  substance  of  the  grain  and  grows  into  a 
hard  blue-black  body  several  times  larger  than  the  grain  (Fig. 
145).  This  body,  known  as  the  sclerotium,  remains  dormant 
during  the  winter  and  in  the  spring  groups  of  hyphae  at  various 
points  in  the  sclerotium  grow  out  into  rose-colored  stalks  that 
terminate  in  globular  heads  (Fig.  146,  C).  These  stalked  bodies 
represent  the  stromata  as  it  appears  in  this  genus.  Numerous 
ascocarps  with  minute  openings  are  developed  in  the  outer  part 
of  globular  head  of  the  stroma  (Fig.  146,  D).  The  ascospores 
germinate  in  the  spring  and  infest  the  flowers  of  the  grain.  The 
Chinese  wonder,  Cordyceps,  is  a  related  parasite  that  attacks 
caterpillars,  larvae  of  beetles  and  truffles.  In  the  case  of  the 
forms  living  upon  insects,  the  parasite  does  not  usually  appear 
until  the  spring,  when  they  are  in  the  pupa  or  cocoon  stage. 
At  this  time,  the  mycelium  which  flourishes  in  the  tissues  of  the 
host,  sends  up  club-like  bodies  (Fig.  147),  that  bear  the  ascocarps 
as  in  the  case  of  the  ergot.  Among  the  important  pests  included 
in  this  order  are  those  causing  the  wilt  disease  of  cotton,  cowpea 
and  watermelon;  the  disease  on  currant,  apple  and  pear  trees; 
and  the  claviceps  of  grain. 


DEVELOPMENT   OF   PLANTS  239 

89.  Order  d.  Sphaeriales  or  Black  Fungi. — This  is  the  largest 
group  of  the  ascomycetes,  over  2,000  species  being  known  in  the 
United  States  alone.  Scarcely  a  fallen  twig  or  bit  of  old  wood 
can  be  examined  without  revealing  the  minute  ascocarps  which 


FIG.  147.  The  stroma  of  Cordyceps  emerging  from  the  pupa  of  a  moth 
and  forming  a  club-like  organ  with  numerous  ascocarps,  as,  in  its  apical 
region. 

more  commonly  are  hard  and  black  in  contradistinction  to  the 
Hypocreales  (Figs.  148;  151,  A).  Many  of  these  fungi  are  quite 
conspicuous  since  the  ascocarps  are  formed  in  large  compact 
masses  and  also  because  they  are  often  associated  with  a  more  or 
less  conspicuous  stroma  (Figs.  149;  151,  D).  The  majority  of 
the  genera  are  saprophytic  upon  dead  and  decaying  vegetation, 
though  some  of  them  are  destructive  parasites.  The  black  knot, 
Plowrightia,  the  cause  of  a  serious  disease  to  plum  and  cherry 
trees,  illustrates  very  well  the  characteristics  of  this  order.  The 
mycelium  grows  in  the  cambium  and  cortical  regions  of  the 
branches,  causing  the  bark  to  split  open  in  the  spring  when  spore 
bearing  hyphae  extend  up  into  the  air  forming  a  velvety  coating 
(Fig.  149,  c).  By  the  approach  of  winter,  this  mycelium  has 
grown  into  the  familiar  black  knotty  mass  in  which  are  de- 
veloped numerous  ascocarp-like  bodies  (Fig.  150).  The  spores 
from  these  ascocarps  are  carried  by  the  wind  in  the  early  spring 
to  other  branches  and  probably  infest  the  budding  trees.  Other 
conspicuous  forms  are  Xylaria  and  Daldinia,  which  develop  an 
extensive  stroma  on  stumps  and  trees  that  contains  numerous 
ascocarps  (Fig.  151).  In  Hypoxylon,  the  stroma  containing  the 


240 


FORMS   OF  THE   BLACK   FUNGI 


ascocarps  breaks  through  the  bark  of  a  large  variety  of  trees 
and  shrubs  in  the  form  of  spherical  or  cake-like  masses  (Fig. 
148).  Among  the  more  serious  pests  may  be  mentioned  the  very 
destructive  black  rot  of  grapes,  apple  and  pear  scab;  bitter  rot 
of  apples;  the  sycamore  blight,  and  the  chestnut  bark  disease. 


FIG.  148.  FIG.  149.  FIG.  150. 

FIG.  148.  A  common  black  fungus,  Hypoxylon:  A,  habit  of  the  fungus 
as  it  appears  on  dead  branches  and  logs.  The  round  black  bodies  are  an 
association  of  the  mycelium,  stroma,  and  numerous  ascocarps.  B,  a  single 
ascus  enlarged,  showing  character  of  the  ascospores. 

FIG.  149.  The  black  knot,  Plowrightia,  infecting  a  branch  of  cherry.  At 
the  bottom  of  the  branch  is  shown  the  early  summer  or  spore-bearing  stage, 
c,  and  above  a  black  warty  mass  of  ascocarps,  as,  produced  the  previous 
season. 

FIG.  150.  A,  several  ascocarps  enlarged,  taken  from  region,  as,  in  Fig. 
149.  B,  diagram  of  an  ascocarp  as  seen  in  section,  showing  the  asci  and  the 
opening  for  the  escape  of  the  ascospores. 

This  and  the  following  order  represent  possibly  the  consumma- 
tion of  those  tendencies  that  we  saw  appearing  in  the  third  line 
of  departure  from  the  Pezizales.  As  stated  they  are  characterized 
by  their  closed  ascocarps. 

90.  Order  e.  Aspergillales  or  Blue-green  and  Brown  Fungi. — 
The  Aspergillales  includes  perhaps  the  most  widely  distributed 
and  familiar  examples  of  the  fungi.  They  occur  as  blue-green  or 
brown  moulds  upon  almost  any  organic  matter,  forming  a  deli- 
cate mycelium  from  which  are  developed  numerous  erect  hyphae. 


DEVELOPMENT   OF   PLANTS 


241 


In  the  common  blue  mould,  Penicillium,  these  erect  hyphae  are 
broom  like  (Fig.  152,  B)  and  the  spores  are  formed  from  the 
tips  of  the  branches  very  much  after  the  manner  of  the  budding 
of  the  yeast  cells.  The  tip  of  a  branch  buds  out  into  a  spherical 
cell  that  is  finally  cut  off  from  the  stalk,  thus  forming  the  spore 
(Fig.  152,  Q.  This  process  is  repeated  again  and  again  just 


FIG.  151.  Other  common  forms  of  the  Spheriales:  A,  habit  of  Hysterio- 
graphium,  on  a  dead  twig.  B,  ascocarps  enlarged.  C,  ascus  enlarged,  show- 
ing character  of  ascospor^s.  D,  Daldinia.  E,  section  of  the  same,  showing 
that  the  stroma  forms  a  concentric  stratum  of  ascocarps,  as,  each  year.  F, 
Xylaria.  G,  the  same  with  branch  cut  off  to  show  the  layer  of  ascocarps 
on  the  periphery  of  the  stroma. 

below  each  successive  spore  and  in  this  way  chains  of  spores 
are  formed.  It  will  be  noticed  that  many  single  spores  are  found 
instead  of  a  large  sporangium  which  contains  many  spores  as 
in  the  case  of  Mucor.  These  spores  are  sometimes  regarded 
as  sporangia  which  have  become  reduced  in  size  and  contain  but 
a  single  spore.  In  Aspergillus,  a  fungus  common  upon  preserves 
and  upon  herbarium  plants  that  have  not  been  sufficiently  dried, 
the  spores  are  developed  from  very  short  branches  that  arise 
from  a  bulbous  swelling  at  the  apex  of  the  erect  hyphae  (Fig. 
152,  A).  The  color  of  the  spores,  as  in  Mucor,  is  the  cause  of 
the  characteristic  blue  or  brown  color  of  the  fungi.  These  ex- 


242    SEXUAL   REPRODUCTION   OF  ASPERGILLALES 

tremely  small  and  numerous  spores  float  off  in  the  air  as  an 
invisible  dust  and  quickly  germinate  under  favorable  conditions, 
forming  new  mycelia. 

The  male  and  female  gametangia  have  departed  considerably 
from  the  form  shown  in  the  cup  fungi.  They  now  appear  as 
short,  slightly  modified  branches  of  the  hyphae  and  have  lost 
the  tubular  outgrowth  so  characteristic  of  the  Pezizales  and  red 


FIG.  152.  Formation  of  spores  or  conidia  in  the  Aspergillales:  A,  spore- 
bearing  hypha  of  Aspergillus.  B,  hypha  of  green  mould,  Penicillium.  C, 
one  of  the  terminal  branches  of  B  enlarged,  showing  manner  of  spore  formation. 

algae.  However  they  become  closely  intertwined  (Fig.  153) 
and  the  ends  of  the  branches  meet,  thus  the  mingling  of  the 
gametes  is  made  possible  through  a  dissolution  of  the  separating 
walls  at  the  tips  of  the  gametangia.  The  resulting  gametospore 
remains  attached  to  the  parent  plant  and  germinates  at  once, 
forming  an  irregular  hyphal  outgrowth  (Fig.  153,  B,  s)  which 
becomes  completely  overgrown  by  the  hyphae  of  the  mycelium 
(Fig.  153,  B,  C).  Thus  is  formed  a  solid  ascocarp  or  perithecium 
(plu.  perithecia),  that  appears  to  the  eye  as  a  minute  grain  of 
sand.  During  this  growth  numerous  lateral  branches  arise  on 
the  hyphae  derived  from  the  gametospore  and  become  trans- 


DEVELOPMENT   OF   PLANTS  243 

formed  into  asci  as  shown  in  Fig.  154,  A,  B.  The  ascocarps 
finally  decay  and  set  free  the  ascospores  which  develop  a  new 
plant  or  mycelium.  Thus  the  entire  life  history  in  these  forms 
as  in  other  fungi  is  suggestive  of  the  algae.  While  conditions  are 


FIG.  153.  Sexual  reproduction  of  the  Aspergillales  and  the  formation  of 
the  ascocarp  or  perithecium :  A,  meeting  of  the  male,  an,  and  female,  o, 
gametangia.  B,  early  stage  in  the  development  of  the  ascocarp.  The  gamet- 
ospore  has  formed  a  series  of  branches,  s,  which  are  being  surrounded  by  hyphae 
(unshaded  in  the  figure)  from  the  mycelium.  C,  later  stage  seen  in  section. 
The  gametospore  has  formed  a  much-branched  body,  s,  which  is  surrounded 
by  a  closely  interwoven  mass  of  hyphae  that  appear  in  section  as  cells. — 
After  Brefeld. 

favorable  for  growth  these  plants  are  rapidly  multiplied  and  dis- 
seminated by  conidia.  Changed  conditions  cause  the  develop- 
ment of  sexual  organs  and  the  consequent  formation  of  the  game- 
tospore. The  gametospore  germinates  at  once,  living  on  the 
parent  plant  like  a  parasite  just  as  in  the  case  of  the  red  algae. 
Note  also  another  resemblance,  the  gametospore  first  develops  a 
mass  of  cells,  in  this  instance  of  a  hyphal  character,  on  which 
later  the  spore-containing  asci  arise  as  lateral  branches. 

The  truffles  are  a  curious  group  of  related  fungi  that  live  for 
the  most  part  entirely  under  ground.  The  mycelium  of  many 
forms  is  supposed  to  live  in  contact  with  the  roots  of  oaks  and 
other  trees  as  a  mycorhiza.  The  fleshy  tuber-like  ascocarp,  often 
as  large  as  a  walnut,  is  a  highly  prized  delicacy  in  Europe  where 
dogs  and  pigs  are  trained  to  locate  the  truffles  by  smell.  This 
industry  amounts  to  more  than  f  1,000,000  annually  in  France 
and  Italy 


244 


THE   POWDERY   MILDEWS 


91.  Order  f.  Perisporiales  or  Powdery  Mildews. — This  name 
is  given  to  a  common  and  widely  distributed  group  of  largely 
parasitic  fungi  that  form  cobwebby  mycelia  on  the  under  surface 
of  the  leaves  of  the  elm,  maple,  willow,  lilac,  rhododendron, 


A 


FIG.  154.  Further  development  of  the  ascocarp:  A,  sectional  view,  show- 
ing the  branches,  s,  derived  from  the  germinating  gametospore,  that  are 
forming  numerous  lateral  branchlets.  B,  one  of  the  branchlets  enlarged, 
showing  how  it  divides  into  cells  which  round  off,  forming  the  asci,  as.  Cf 
ascospore.  D,  germinating  ascospore. — After  Brefeld. 

Virginia  creeper,  etc.  (Fig.  155),  and  a  great  variety  of  herba- 
ceous plants,  as  the  dandelion  and  cocklebur.  The  majority  of 
them  do  not  seriously  interfere  with  the  health  of  the  plant,  but 
others  produce  serious  diseases  in  young  cherry  and  plum  trees, 
hop  vines,  gooseberry,  etc.  These  parasites  are  external  and 
obtain  their  food  by  means  of  short  branches,  haustoria,  which 
dissolve  the  cell  wall  and  absorb  the  cell  contents  as  shown  in 
Fig.  156,  h.  The  members  of  this  order  are  of  special  interest 
in  that  they  throw  considerable  light  upon  the  nature  of  para- 
sitism. Frequently  one  and  the  same  form  will  grow  upon  a 
wide  variety  of  plants,  just  as  we  will  see  to  be  the  case  in  another 
group,  i.  e.,  the  rusts.  Erysiphe  gmminis,  for  example,  is  found 
upon  barley,  wheat,  rye,  oats  and  several  genera  of  grasses. 
But  the  form  growing  upon  any  one  of  these  kinds  of  plants  will 


DEVELOPMENT   OF   PLANTS 


245 


not  infest  any  of  the  others.  It  would  seem  that  the  parasite 
becomes  changed  and  specialized,  although  there  is  no  visible  evi- 
dence of  this,  and  so  after  a  time  is  able  to  live  upon  but  one 
kind  of  plant.  These  specialized  forms  are  termed  biological 
species,  in  contradistinction  to  the  general  or  morphological 
species  which  include  them  all.  Evidently  there  is  something 
within  the  plant  that  not  only  attracts  these  parasites,  but  also 
changes  them,  so  that  after  a  time  they  can  only  live  upon  the 


FIG.  155.  FIG.  156. 

FIG.  155.  Appearance  of  one  of  the  powdery  mildews,  Uncinula,  on  leaf 
of  elm. 

FIG.  156.  Enlarged  view  of  the  mycelium,  ascocarp,  etc.,  of  one  of  the 
mildews,  Erysiphe:  c,  erect  hyphae  forming  spores  or  conidia;  h,  haustoria 
penetrating  epidermis  of  leaf;  a,  ascocarp  or  perithecium. 

plant  having  these  substances.  If  this  material  is  absent  from 
the  plant  then  it  is  immune  and  it  has  been  shown  in  a  few  cases 
where  individual  plants  were  not  subject  to  a  plant  disease  that 
this  was  due  to  the  lack  of  a  substance  which  the  infested  plants 
had  or  to  the  presence  of  a  new  substance  which  was  repellent  to 
the  parasite.  The  relation  between  parasite  and  host  is  strikingly 
brought  out  by  Massee's  experiment,  in  which  he  claims  that  a 
purely  saprophytic  fungus  was  induced  to  become  a  destructive 
parasite  upon  the  leaves  of  a  species  of  Begonia  by  injecting  the 
leaves  with  a  sugar  solution.  The  fungus  flourished  upon  the 
leaves  treated  in  this  way  and  produced  spores.  These  spores 
were  sown  upon  leaves  similarly  treated  and  this  was  repeated  for 


246 


REPRODUCTION   OF   PERISPORIALES 


twelve  generations,  at  which  time  he  states  that  the  spores  would 
grow  upon  the  untreated  leaves.  Perhaps  this  is  the  explanation 
of  some  epidemics  or  the  occasional  sudden  appearance  of  a  plant 
disease.  A  variety  of  circumstances  might  cause  plants  to  form 
substances  attractive  to  the  parasite  or  to  fail  to  develop  repellent 


D 


FIG.  157.  Sexual  reproduction  of  a  powdery  mildew:  A,  meeting  of  the 
male,  an,  and  female,  o,  gametangia.  B,  fertilization,  the  male  gamete,  mt 
is  seen  approaching  the  female.  C,  section  of  young  ascocarp  showing  the 
early  germination  of  the  gametospore,  which  has  become  surrounded  by 
hyphae  derived  from  the  mycelium.  D,  later  stage,  the  gametospore  has 
developed  several  cells  and  the  second  cell  from  the  end,  as,  will  produce 
the  ascus. — After  Harper. 

materials.  In  either  case  they  would  become  susceptible  to  the 
disease. 

The  spores  are  formed  in  chains  (Fig.  156,  c)  from  the  end 
of  the  erect  hyphae  that  project  from  the  surface  of  the  leaf  in 
thick  masses,  causing  the  powdery  appearance  and  the  popular 
name  of  these  parasites.  These  spores  germinate  quickly  and 
rapidly  spread  the  fungus. 

The  reproductive  organs  have  in  this  group  become  still 
further  reduced  and  appear  as  short  branches  as  shown  in  Fig. 
157,  A.  The  solution  of  the  walls  at  the  point  of  contact  of 
these  organs  permits  the  male  gamete  to  pass  over  and  fuse 
with  the  female  (Fig.  157,  B).  The  growth  of  the  gametospore 
forms  a  limited  number  of  cells  and  one  of  them,  usually  the 


DEVELOPMENT   OF   PLANTS 


247 


second  from  the  end,  will  develop  one  or  several  asci  (Fig.  157, 
Cj  D).  As  in  Penicillium,  this  growth  becomes  enveloped  by  a 
mass  of  hyphae  that  originate  from  the  hyphae  bearing  the 
gametospore  and  from  adjacent  strands  of  the  mycelium.  These 
ascocarps  appear  at  maturity  as  black  specks  and  in  the  majority 
of  forms  they  are  provided  with  hair-like  outgrowths  that  are 
very  regular  and  characteristic  of  the  genera  (Fig.  158).  The 
ends  of  these  hairs  are  rather  mucilaginous  and  may  assist  in 
the  dissemination  of  the  ascocarps.  The  ascospores  are  resting 


FIG.  158.  Forms  of  ascocarps  found  among  the  powdery  mildews:  A, 
Phyllactinia  with  needle-like  appendages  enlarged  at  the  base.  B,  Micro- 
sphaera,  appendages  dichotomous  at  apex.  C,  Uncinula,  appendages  coiled 
at  apex.  D,  Erysiphe  without  appendages  and  crushed  to  show  escaping 
asci.  E,  an  ascus  containing  six  ascospores. 

spores  adapted  to  enduring  drought  and  cold  as  in  Penicillium, 
which  they  resemble  in  their  discharge  and  germination. 

92.  Reduced  Ascomycetes. — Space  will  only  permit  the 
consideration  of  two  other  groups  from  this  enormous  alliance 
of  the  Ascomycetes.  These  forms  are  here  considered  partly 
because  of  their  economic  importance  and  partly  because  they 
illustrate  the  reduction  that  may  go  on  in  the  plant  body.  These 
two  groups  of  fungi  have  become  so  changed  and  simplified  in 
their  structure  and  life  history  as  to  render  impossible  a  guess 
as  to  their  relationship  to  other  forms. 


248 


REDUCED  ASCOMYCETES 


Q2A.  Order  g.  Exoascales  or  Peach  Curl.— This  group  in- 
cludes a  small  number  of  parastic  fungi  that  are  especially  destruc- 
tive to  peach  and  plum  trees,  causing  distortion  of  the  leaves 
and  fruit,  known  as  leaf  curl  and  bladder  plum  (Fig.  159).  There 
is  apparently  no  trace  of  a  reproductive  process,  the  mycelium 
spreading  through  the  leaf  develops  directly  a  rudimentary 
hymenium  of  numerous  asci  beneath  the  cuticle,  which  is  finally 


FIG.  159.  FIG.  1 60. 

FIG.  159.  A  branch  from  peach  tree,  showing  the  distortion  of  the  leaves 
caused  by  the  fungus,  Exoascus. 

FIG.  1 60.  Section  of  a  leaf  showing  numerous  cells  rupturing  the  cuticle 
and  developing  into  asci,  as. 

ruptured  by  their  growth  (Fig.  160).  The  ascospores  are  dis- 
charged into  the  air  by  the  bursting  of  the  asci  and  carried  by 
the  wind  to  other  plants.  The  damage  in  the  United  States  to 
peach  trees  alone  is  estimated  at  $2,000,000  to  $3,000,000 
annually. 

93.  Order  h.  Yeast  or  Saccharomycetes. — These  fungi  are 
unicellular  plants  with  little  suggestion  of  mycelial  growth. 
Ordinarily  they  consist  of  rather  oval  cells  which  multiply  rapidly 
by  a  budding  suggestive  of  conidia  formation  in  preceding  orders 
(Fig.  161).  The  new  cells  that  push  out  from  the  side  of  the 
mother  cell  readily  drop  off,  but  in  rapid  growth  they  may 
remain  attached  in  chains  (Fig.  161,  D).  Under  certain  condi- 
tions, as  the  exhaustion  of  the  food  supply,  the  cells  become 
transformed  into  asci  and  the  contents  of  each  cell  rounds  off 
into  one  or  more  ascospores  (Fig.  161,  E).  The  ascospores  are 


DEVELOPMENT   OF   PLANTS 


249 


freed  by  the  decay  of  the  ascus  and  when  conditions  are  favorable, 
grow  into  the  characteristic  yeast  cells,  as  shown  in  Fig.  161,  F. 

(a)  Fermentation. — These  microscopic  plants  must  be  num- 
bered among  those  plants  that  are  of  the  greatest  economic  value . 
Their  importance  is  due  to  the  fact  that  they  decompose  sugars 
upon  which  they  feed  into  CO2  and  alcohol,  a  change  called  fer- 
mentation. The  extensive  brewing  and  distilling  industries  all 


FIG.  161.  The  yeast  plant,  Saccharomyces:  A,  single  plant,  B,  plant 
producing  three  buds.  C,  section  of  two  plants  showing  buds  and  nuclear 
division.  D,  chain  of  plants  due  to  rapid  budding  and  growth.  E,  forma- 
tion of  ascospores.  F,  germination  of  an  ascospore  and  the  formation  of 
new  plants  by  budding. — After  Wager. 

over  the  world  are  dependent  upon  the  growth  and  peculiar  action 
of  these  microscopic  plants.  When  yeast  plants  are  placed  in 
solutions  containing  sugar  in  the  form  of  molasses  or  prepara- 
tions of  rye,  corn,  barley,  potatoes,  etc.,  and  slightly  warmed,  the 
growth  of  the  yeast  produces  a  vigorous  fermentation.  Carbon 
dioxide  rises  to  the  surface,  forming  a  frothy  scum  while  the  alco- 
hol accumulates  in  the  fluid.  Beers  and  ales  are  fermented  bever- 
ages of  this  nature,  while  whiskies,  brandies,  alcohol,  etc.,  are 
obtained  from  the  fermented  mass  by  removing  a  part  of  the 
water  by  distillation.  Wines  and  cider  are  weak  alcoholic  bev- 
17 


250  THE   NATURE   OF  YEAST 

erages  formed  by  the  yeast  plant  from  the  sugars  in  the  juices 
expressed  from  grapes  and  apples.  In  this  case,  the  yeast  is 
generally  allowed  to  find  its  way  naturally  into  the  fluids.  So 
minute  are  these  plants  that  they  are  distributed  in  the  air 
throughout  the  world  and  no  weak  sugar  solution  can  be  exposed 
for  any  length  of  time  without  being  inoculated  by  them — a  fact 
that  has  been  apparently  taken  advantage  of  by  man  in  all  times 
and  places.  When  these  fermented  beverages  are  bottled  before 
the  decomposition  of  the  sugar  is  complete,  then  a  further  gen- 
eration of  CO2  sets  up  a  pressure  in  the  bottle  or  cask  that  causes 
the  popping  of  the  cork  when  the  bottle  is  opened  and  the  sparkle 
of  the  fluid  owing  to  the  escaping  gas.  These  forms  show  the 
same  range  of  variability  in  the  work  which  they  perform  as 
noted  in  the  bacteria  of  decay.  In  the  case  of  cider  our  govern- 
ment experts  have  separated  several  forms  of  yeast  that  are  now 
offered  for  distribution  and  which  give  distinctive  flavors  to 
cider.  So  in  brewing  and  bread  making  enough  has  been  done 
to  show  that  there  are  numerous  forms  of  yeast  that  differ 
materially  in  their  power  to  induce  fermentation  as  well  as  in 
the  nature  of  the  fermentations  that  they  bring  about. 

(&)  Bread  Making. — The  most  important  use  of  the  yeast  plant 
is  its  application  to  the  "raising"  of  bread.  Flour  contains,  in 
addition  to  starch,  a  little  sugar,  and  this  amount  is  increased  in 
bread  making  by  a  ferment,  diastase,  in  the  flour  which  changes 
starch  into  sugar.  The  flour  is  mixed  with  water  containing 
yeast  plants  into  a  dough  and  placed  in  a  warm  place,  when  it 
begins  to  rise.  This  means  that  the  yeast  plants  begin  to  grow 
and  decompose  the  sugar  into  CO2  and  alcohol.  The  dough 
prevents  the  escape  of  the  gas  which  collects  in  bubbles  in  the 
dough,  causing  it  to  swell.  Baking  further  expands  the  gas  and 
also  drives  off  the  water  and  alcohol,  leaving  the  bread  light  and 
porous.  There  are  many  forms  of  yeast  plants  which  differ  in 
their  power  of  producing  fermentations  and  in  the  flavor  which 
they  impart.  Consequently,  different  forms  are  used  for  differ- 
ent purposes.  Bread  yeast  is  a  form  that  has  been  selected  be- 
cause of  the  quickness  of  its  action  and  the  flavor  that  it  imparts 
to  bread.  Bread  yeast  is  grown  in  large  vats  and  put  up  with 
starch  in  cakes,  known  as  compressed  yeast. 


DEVELOPMENT   OF   PLANTS  251 

CLASS  C.     BASIDIOMYCETES  OR  BASIDIA-FORMING  FUNGI 

94.  General  Features. — This   class  contains  two   important 
groups  of  fungi;  one  of  which  includes  very  destructive  para- 
sites, and  the  other  comprises  those  conspicuous  and  familiar 
saprophytes  known  as  mushrooms,   bracket  fungi,   puff  balls, 
etc.     The  striking  feature  of  the  class  is  the  formation  of  spores 
on  club-like  hyphae  called  basidia  (sing,  basidium)  (Fig.  171,  D). 
These  organs  are  often  developed  in  rather  complex  outgrowths 
of  the  mycelium  which  may  be  fleshy  or  woody,  as  in  the  mush- 
room and  bracket  fungi  (Figs.  170;  173,  B).     There  is  no  known 
sexual  reproduction,  save  possibly  in  the  first  order  mentioned 
below  where  a  vegetative  union  of  cells  appears  to  have  been 
substituted  for  a  fertilization  process  which  was  originally  like 
that  seen  in  the  red  algae.     The  more  important  orders  are  the 
following : 

95.  Order  a.     Uredinales  or  Rusts. — These  parasites  are  well 
known  by  the  streaks  and  blotches  of  yellow  or  black  rust  which 
they  produce  on  the  leaves  and  stems  of  a  great  variety  of  plants. 
About  2,000  species  are  known  in  the  United  States.     They  are 
among  the  most  destructive  parasites,  causing  great  damage  to 
wheat,  oats,  apples,  quinces,  roses,  carnations,  etc.     The  yearly 
loss  from  grain  rust  alone  is  estimated  at  considerably  over 
$18,000,000  in  the  United  States.     They  exhibit  a  degree  of 
variation  not  paralleled  among  other  plants,  more  than  five  dif- 
ferent kinds  of  spores  being  formed  by  some  species  in  their 
life  history.     This  is  due,   doubtless,  to  the  influence  of  the 
climate,  the  spores  varying  with  the  season  (spring,  summer  and 
fall)  and  the  plants  upon  which  the  fungus  grows,  for  one  and  the 
same  fungus  may  grow  upon  different  plants,  producing  one  or 
more  kinds  of  spores  on  each.     Forms  infesting  different  species 
of  plants  are  said  to  be  heteroecious  and  those  living  upon  but  one 
species  are  termed  autoecious. 

(a)  The  Life  History  of  Puccinia. — Several  species  of  this 
genus  infest  wheat  and  illustrate  the  many  forms  that  may 
appear  in  the  life  history  of  a  rust.  One  phase  of  the  life  of  this 
parasite  appears  upon  the  leaves  of  the  barberry.  During  May 
and  June  the  mycelium  growing  in  the  leaves  forms  roundish 


252  DEVELOPMENT   OF  A   RUST 

bodies  which  rupture  the  epidermis  and  finally  open  out  into  cups 
filled  with  chains  of  yellowish  spores.  An  examination  of 
Fig.  162  shows  that  these  spores  are  formed  in  rows  at  the  end  of 
hyphae  and  surrounded  by  a  layer  of  rather  thick-walled  hyphae. 
This  stage  of  the  rust  is  known  as  the  cluster  cup  or  aecial  stage 
and  the  spores  are  called  aeciospores.  Often  smaller  spore-bear- 


FIG.  162.  Cluster  cups  as  seen  in  section  of  leaf  of  spring  beauty,  Clay- 
tonia.  At  right  one  of  the  cups  is  ruptured,  exposing  the  aeciospores. 
Below  a  small  cup,  pycnium,  is  discharging  pycniospores  that  are  possibly 
functionless  male  gametes. 

ing  cups,  known  as  pycnia  (sing,  pycnium),  are  associated  with 
this  phase  of  the  fungus.  These  small  spores,  while  capable  of 
germinating,  do  not  appear  to  enter  into  the  life  history  of  the 
fungus  by  producing  a  new  parasite.  They  have  been  looked 
upon  as  male  gametes  that  originally  effected  fertilization  in  a 
female  organ  from  which  developed  the  spore-bearing  cluster  cup, 
the  process  being  similar  to  that  noted  in  the  Red  Algae.  On 
the  other  hand  they  are  regarded  by  some  authorities  as  spores 
that  have  lost  their  power  to  germinate  and  so  breed  the  fungus 
asexually.  The  process  of  spore  formation  in  the  cluster  cup, 
as  will  be  seen  from  the  outline  given  below,  is  so  different  from 
that  of  the  algae  and  is  known  in  so  few  forms  that  any  interpre- 
tation of  the  phenomena  should  be  deferred  for  the  present. 


DEVELOPMENT   OF   PLANTS 


253 


The  spore-producing  hyphae  in  the  young  aecidia  form  a  rather 
loose  stratum  of  somewhat  elongated  cells  (Fig.  162,  A ,  i) .  These 
cells  divide,  forming  an  upper  series  of  cells  which  are  smaller 
and  narrower  than  the  lower  or  basal  cells.  These  upper  cells  are 
sterile,  ultimately  disappearing,  and  they  have  been  compared 


FIG.  162  A.  Spore  formation  in  a  cluster  cup:  i,  appearance  of  the  hyphae 
in  a  young  cup.  2,  one  of  the  cells  from  Fig.  i,  after  it  has  divided  into 
the  sterile  cell  and  the  large  basal  cell.  3,  early  stage  in  the  fusion  of  the 
basal  cells  of  two  adjacent  hyphae.  4,  the  two  nuclei  of  the  fused  cells  have 
divided  and  two  of  them  are  passing  to  the  base  of  the  cells  and  two  into  the 
fused  region  of  the  cells.  5,  the  upper  portion  of  the  fused  area  is  cut  off, 
forming  the  spore  mother  cell.  This  cell  will  divide  once,  forming  a  spore 
and  a  small  sterile  cell.  6,  a  chain  of  spores  and  sterile  cells  is  being  formed 
as  noted  in  5. — After  Christman. 

to  remnants  of  the  tubular  outgrowth  of  the  female  gametangium 
noted  in  the  red  algae. 

The  basal  cells  enlarge,  become  inclined  towards  one  another 
in  pairs  and  finally  meet  at  their  upper  ends,  the  walls  dissolving 
at  the  point  of  contact.  The  nuclei  of  the  two  cells  now  pass 
to  the  fused  region  of  the  cells  and  divide,  forming  four  nuclei. 
Two  of  these  wander  back  into  the  base  of  their  respective  cells, 
while  the  other  two  pass  to  the  upper  portion  of  the  fused  area 


254       FORMATION   OF  SPORES   IN  THE    RUSTS 

and  become  separated  from  the  basal  nuclei  by  a  transverse 
wall  (Fig.  162,  A,  3-5).  The  upper  cell  formed  in  this  manner  is 
the  spore  mother  cell  and  it  divides  at  once,  forming  an  aeciospore 
and  a  small  intercalary  cell.  As  soon  as  this  process  is  completed 
the  two  nuclei  at  the  base  of  the  fused  cells  move  up  again  into 
the  fused  portion  of  the  cells,  which  region  has  elongated  in  the 
meantime,  and  divided  as  before.  In  this  way  a  chain  of  binu- 
cleate  aeciospores  is  formed,  separated  by  small  intercalary  cells 
(Fig.  162,  A,  6)  that  soon  disintegrate. 

As  soon  as  the  cluster  cups  are  ruptured  the  aeciospores  are 
scattered  by  the  wind,  and  singularly,  will  only  germinate  and 
infest  the  seedlings  and  young  leaves  of  the  wheat  in  which  they 
develop  a  mycelium  similar  to  that  of  the  barberry,  but  note  that 
the  cells  of  this  mycelium  are  binucleate  like  the  aeciospores. 
However,  this  mycelium  forms  in  place  of  cluster  cups,  groups  of 
erect  hyphae  at  the  ends  of  which  single  reddish  spores  are  formed 
(Fig.  163,  C).  This  growth  ruptures  the  epidermis,  exposing 
the  spores  in  rusty  lines  of  blotches  (Fig.  163,  J3).  These  yellow- 
brown  blotches  account  for  the  popular  name  of  Rusts  given  to 
this  group  of  fungi.  This  is  the  summer  or  uredinal  stage  of  the 
parasite.  The  spores  are  known  as  urediniospores.  This  phase 
of  the  fungus  is  a  very  destructive  one,  for  the  spores  are  formed 
in  great  numbers  and  provided  with  thin  walls.  They  are  widely 
distributed  by  the  wind  and  germinate  at  once  (Fig.  164,  A) 
on  other  wheat  plants  which  soon  show  the  rusty  brown  streaks 
of  urediniospores.  In  this  way,  the  pest  spreads  with  great  rap- 
idity from  a  single  center  of  infection.  This  formation  of  ure- 
diniospores goes  on  during  the  summer,  sapping  the  vitality  of 
the  plant  and,  in  severe  cases  of  infection,  materially  interferes 
with  the  maturing  of  the  grain. 

Later  in  the  season,  this  same  mycelium  forms  in  the  leaves 
of  the  wheat  quite  a  different  type  of  spore.  They  are  formed  in 
the  same  manner  as  the  urediniospores  but  are  provided  with 
thick  dark  walls  and  from  one  to  several  spores  are  developed  at 
the  end  of  the  hyphae  (Fig.  163,  Z>).  Consequently,  when  the 
epidermis  is  ruptured,  these  spores  form  rusty  black  blotches  on 
the  leaves.  This  third  stage  is  known  as  the  telial,  since  it  ends 


DEVELOPMENT   OF   PLANTS 


255 


the  season's  growth.  These  spores,  called  teliospores,  are  resting 
spores  and  tide  the  fungus  over  the  winter.  Furthermore  these 
spores  are  uninucleate.  The  two  nuclei  which  have  character- 
ized all  cells  from  the  aecial  stage  on,  actually  fuse,  forming  one 
nucleus  as  the  teliospores  mature.  Some  regard  this  as  a  delayed 
fertilization.  The  teliospores  germinate  in  the  spring  quite 


FIG.  163. 


FIG.  164. 


FIG.  163.  The  summer  and  fall  stages  of  a  rust,  Puccinia:  A,  rust  blotches 
on  leaf  of  wheat.  B,  portion  of  leaf  magnified,  showing  rupturing  of  the 
epidermis  due  to  the  formation  of  spores.  C,  urediniospores  or  the  summer 
spores  which  effect  a  rapid  distribution  of  the  parasite  during  the  summer. 
D,  teliospores  or  fall  spores  which  are  dormant  during  the  winter. 

FIG.  164.  Germination  of  uredinio-  and  telio-spores:  A,  the  thin- walled 
urediniospore  sending  out  hyphae  from  thin  places  in  its  wall.  This  is  ef- 
fected as  soon  as  it  is  carried  by  the  wind  to  a  moist  leaf.  B,  teliospore  ger- 
minating in  the  spring  and  forming  a  short  hypha,  from  the  end  of  which  four 
cells  have  been  cut  off  that  are  forming  the  basidiospores,  b. 

independent  of  any  plant  and  being  dependent  upon  the  food 
stored  in  the  spore,  they  only  form  short  hyphae  which  usually 
become  divided  into  four  cells  (Fig.  164,  B).  This  structure  is 
known  as  tlie  basidium.  Each  cell  of  the  basidium  sends  out  a 
delicate  tube  into  the  end  of  which  the  cell  contents  passes,  thus 


256  FORMS   OF  THE   RUSTS 

forming  a  small  spore,  known  as  the  basidiospore.  This  basidial 
stage  completes  the  life  history  of  the  fungus  for  the  basidiospores 
are  carried  to  the  leaves  of  the  barberry  and  begin  again  the  life 
cycle  of  the  parasite  by  forming  the  cluster  cups.  It  is  held  by 
many  that  this  sequence  in  spore  formation  represents  an  alter- 
nation of  generation.  According  to  this  view  the  aecium  is  the 
result  of  a  sexual  process.  Therefore  the  aeciospore  would  be 
the  beginning  of  the  sporophyte  or  asexual  generation,  which 
terminates  in  the  teliospore.  The  germination  of  the -teliospore 
marks  the  beginning  of  the  sexual  generation  and  it  is  interesting 
to  note  that  we  have  four  cells  produced  as  in  the  tetraspores  of 
the  red  algae. 

It  is  not  surprising  that  this  story  was  not  unravelled  for  a 
long  time  and  that  these  different  stages  of  the  parasite  were 
known  as  distinct  species.  The  first  clue  to  the  relationship  was 
gained  in  England  where  it  was  observed  that  the  wheat  fields 
to  the  leeward  of  the  barberry  bushes  were  especially  infested  with 
rust.  For  this  reason,  a  law  was  passed  early  in  the  history  of 
Massachusetts  compelling  the  destruction  of  the  barberry  bushes. 
This  suggestion  of  relationship  between  the  cluster-cup  stage  of 
the  barberry  and  the  rust  of  the  wheat  finally  led  to  the  inocu- 
lation of  wheat  plants  with  aeciospores  and  this  resulted  after 
a  week  or  more  in  the  appearance  of  the  characteristic  rusty 
streaks  on  the  leaves  of  the  wheat. 

(b)  Features  in  the  Life  History  of  Other  Rusts. — Considerable 
variation  characterizes  these  rusts,  not  all  of  them  having  so 
elaborate  a  life  history  as  that  outlined  above.  One  of  the  most 
common  species  of  Puccinia  affecting  the  wheat  is  perennial  in 
the  wheat  and  possibly  in  other  grasses  where  it  produces  ure- 
diniospores  in  the  spring,  thus,  the  aecial,  telial  and  basidial 
stages  are  eliminated.  In  the  apple  rust,  the  uredinial  stage  is 
missing.  This  disease  produces  on  the  leaves  of  various  members 
of  the  apple  family,  yellow  patches  in  which  are  formed  tube-like 
cluster  cups  (Fig.  165,  A}.  The  aeciospores  are  only  capable  of 
infesting  the  juniper,  in  the  branches  of  which  they  produce 
gall-like  swellings  (Fig.  165,  B)  known  as  cedar  apples  and  also 
sometimes  bushy  outgrowths  known  as  witches'  brooms.  In  the 


DEVELOPMENT   OF   PLANTS 


257 


spring,  variously  shaped  masses  of  hyphae  bearing  numerous 
teliospores  radiate  out  from  these  galls  and  in  the  early  spring 
rains  these  strands  swell  up,  forming  conspicuous  yellow,  jelly- 
like  masses.  The  basidiospores  are  developed  in  the  jelly  and 
infest  the  leaves  of  the  apple,  thorn,  shadbush,  etc.  In  some 
of  the  genera  of  rusts  all  the  stages  appear  upon  the  same  plant 
as  in  the  May  apple  and  jack-in-the-pulpit.  In  the  early  spring 


FIG.  165.  A  rust,  Gymno sporangium,  that  infests  the  juniper  and  mem- 
bers of  the  apple  family:  A,  cluster  cups  on  leaf  of  thorn  apple.  B,  telio- 
spore  stage  on  red  cedar,  juniper. 

the  stems  and  leaves  may  often  be  seen  infected  with  the  yellow 
cluster  cups  which  are  followed  later  by  the  dark-colored  telio- 
spores. In  other  cases,  teliospores  only  are  produced,  as  in  the 
hollyhock,  and  this  was  doubtless  the  original  form  of  the  spore. 
Numerous  physiological  species  have  been  reported  among  the 
rusts  as  in  the  case  of  the  powdery  mildews,  and  in  both  groups 
we  find  some  of  these  specialized  forms  able  to  grow  upon  differ- 
ent plants  if  an  intermediate  plant  is  used  to  "bridge"  them  over. 
Thus  a  physiological  species  that  grows  normally  upon  the  oat 
but  not  upon  rye  or  wheat  may  be  made  to  do  so  by  first  growing 
it  upon  barley.  Spores  derived  from  the  barley  will  now  infest 
rye  or  wheat. 

96.  Order  b.  Ustilaginales  or  the  Smuts. — These  parasites 
are  more  primitive  than  the  rusts  to  which  they  are  doubtless 


258 


FORMS   OF  SMUTS 


related.  They  produce  very  generally  but  one  form  of  spore  that 
may  possibly  be  compared  to  the  teliospore.  They  are  com- 
mon and  exceedingly  destructive  parasites,  affecting  especially 
the  flowers  and  fruits  of  corn  and  other  cereals  as  wheat,  oats 
and  barley  (Fig.  166).  The  damage  by  smuts  to  the  corn  crop 


FIG.  1 66. 


FIG.  167. 


FIG.  166.  A  common  smut,  Ustilago,  transforming  the  kernels  of  corn 
into  sooty  black  pustules. 

FIG.  167.  The  formation  and  germination  of  the  spore  of  a  smut:  A, 
the  formation  of  the  spores  from  the  mycelium  in  the  kernel  of  corn.  B, 
germination  of  a  spore  and  the  appearance  of  the  basidiospore. 

of  the  United  States  exceeds  $2,000,000  annually.  In  the  case 
of  corn,  the  parasite  keeps  pace  with  the  growth  of  the  plant 
without  producing  serious  damage  until  the  flowers  appear,  when 
the  mycelium  increases  greatly  in  the  affected  ears  and  "tassels" 
producing  sometimes  enormous  malformation,  especially  in  the 
ears,  which  appear  as  glistening  white  blisters  or  pustules.  Later, 
these  bodies  change  to  a  sooty  black,  owing  to  the  transformation 
of  the  cells  of  the  mycelium  (Fig.  167,  A)  into  black  greasy  spores. 
These  spores  are  scattered  in  clouds  upon  the  breaking  of  the 
pustules  and  germinate  much  after  the  manner  of  the  telio- 
spores  of  the  rusts,  infesting  the  young  and  delicate  parts  of 


DEVELOPMENT   OF   PLANTS  259 

other  corn  plants  by  means  of  their  basidiospores  (Fig.  167,  B). 
In  other  cases,  the  teliospores  germinate  in  the  spring,  infesting 
the  seedlings.  This  appears  to  be  always  the  case  in  the  de- 
structive wheat,  rye,  oat  and  barley  smuts.  Infection  only 
takes  place  when  the  spores  come  in  contact  with  the  seed.  Con- 
sequently, in  these  cases,  the  fungus  can  more  readily  be  fought 
by  treating  the  seed  with  some  fungicide  as  formaline  and  copper 
sulphate. 

97.  Order  c.  Agaricales  or  Mushrooms  and  Toadstools. — 
This  is  the  most  familiar  group  of  the  basidiomycetes  and  its 
12,000  odd  species  are  Commonly  referred  to  as  mushrooms  and 
toadstools.  These  fungi  are  largely  saprophytes,  living  upon 
the  humus  in  the  soil  and  upon  decaying  wood.  Several  of  the 
genera  are  exceedingly  destructive  to  trees  and  cause  great  loss 
to  the  lumber  industry.  In  the  majority  of  cases,  the  fungi 
appear  unable  to  attack  the  living  portion  of  the  trees  and  only 
thrive  upon  dead  tissues,  as  the  heart  wood,  which  they  quickly 
disorganize  and  render  worthless.  As  in  all  the  preceding  groups 
of  fungi,  the  real  plant  body  is  a  delicate  mycelium  that  spreads 
through  the  soil  or  decaying  substances,  as  may  readily  be  dem- 
onstrated by  splitting  open  a  tree  infested  with  one  of  the  fungi 
(Fig.  1 68).  The  "spawn"  that  is  sold  in  seed  stores  for  plant- 
ing in  mushroom  beds  is  a  dried  mass  of  decaying  leaves  and 
straw  mixed  with  earth,  in  which  the  mycelium  of  the  mushroom 
has  been  allowed  to  grow.  Often  the  hyphae  of  the  mycelium 
lose  their  delicate  character  and  become  woven  together,  forming 
rather  dense  woody  strands  or  plates,  a  development  frequently 
seen  in  the  timbers  of  mines  and  under  the  bark  of  decaying  trees. 
In  certain  species  the  mycelium  is  phosphorescent  and  the  cause 
of  the  pale  light  that  sometimes  appears  upon  moist  decaying 
wood,  the  so-called  fox  wood.  The  mycelium  bears  at  various 
places  the  complex  fleshy  or  woody  body  commonly  known  as 
the  mushroom  or  toadstool.  If  the  mycelium  grows  in  a  regular 
manner,  radiating  outward  in  all  directions,  then  the  mushrooms 
will  have  a  similar  arrangement.  The  older  central  portions  of 
the  mycelium  will  finally  die  off,  while  the  newer  portions  con- 
tinue to  radiate  outward  and  produce  the  mushrooms,  which  con- 


26o 


DEVELOPMENT   OF  A   MUSHROOM 


sequently  appear  in  a  more  or  less  regular  circle.  In  this  way, 
the  fairy  rings,  which  are  often  held  in  superstitious  awe,  come 
about. 

The  Mushroom  or  Toadstool. — The  common  umbrella  type  of 
mushroom  consists  of  a  stalk  or  stipe  and  a  cap  or  pileus,  on 


FIG.  1 68.  FIG.  169. 

FIG.  1 68.  The  mycelium  of  one  of  the  Agaricales  forming  white  masses 
as  its  spreads  through  wood. 

FIG.  169.  Development  of  a  mushroom:  3,  early  appearance  of  the  mush- 
room as  a  ball  of  hyphae  on  the  strands  of  the  mycelium.  I,  section  of  one 
of  these  spherical  masses  of  hyphae,  showing  the  circular  openings  in  which 
the  gills  are  developed.  2,  a  later  stage  with  the  gills  formed  and  the  velum, 
vl,  appearing  asa  delicate  membrane. 

the  underside  of  which  are  located  radiating  plates  or  gills  (Fig. 
170,  A}.  This  structure  originates  on  a  strand  of  the  mycelium 
as  a  very  small  ball  composed  of  a  mass  of  interwoven  hyphae 
(Fig.  169,  3).  Soon,  however,  the  hyphae  of  this  mass  begin 
to  grow  in  a  very  regular  manner.  At  first  the  growth  results 
in  the  formation  of  a  cavity  in  the  upper  part  of  the  ball  that 
extends  completely  around  it  (Fig.  169,  i).  Hyphae  now  grow 


DEVELOPMENT   OF   PLANTS 


261 


down  into  this  cavity  in  regular  lines,  forming  the  radiating 
plates  or  gills  noted  in  Fig.  170.  This  development  divides  the 
ball  of  hyphae  into  an  upper  part  or  pileus  and  a  basal  region, 
the  stalk  or  stipe.  As  this  growth  proceeds  the  mass  of  hyphae 
extending  from  the  margin  of  the  pileus  to  the  stipe  becomes 

P 


FIG.  170.  Habit  of  a  poisonous  mushroom,  Amanita:  A,  the  mature 
mushroom — s,  stipe;  p,  pileus;  g,  gills;  a,  annulus;  v,  volva,  a  part  of  which 
appears  in  patches  on  the  top  of  the  pileus.  C,  young  form  of  the  Amanita, 
the  volva  beginning  to  break.  D,  later  development,  the  volva  completely 
ruptured,  disclosing  the  pileus,  stipe  and  velum,  vl. — H.  O.  Hanson. 

drawn  out  and  ultimately  forms  a  rather  thin  membrane  known 
as  the  velum  (Fig.  169,  2,  vl).  This  entire  growth  is  often 
enveloped  by  a  sheath-like  mass  of  hyphae,  the  volva  (Fig.  170, 
C,  D).  At  this  stage  of  development,  the  young  mushroom  is 


262  GROWTH   OF  A   MUSHROOM 

rather  spherical  in  shape  and  so  small  that  it  is  quite  concealed 
in  the  ground  or  by  surrounding  vegetation.  When  conditions 
are  favorable  for  further  growth,  as  after  a  rain,  each  cell  of  this 
miniature  fungus  absorbs  moisture  and  rapidly  expands,  thus 
causing  the  mushroom  to  spring  up  as  by  magic  and  reach  its 
full  growth  in  a  few  hours.  As  the  stipe  elongates,  the  pileus 
spreads  out  like  an  umbrella,  rupturing  the  veil,  a  part  of  which 
clings  to  the  stalk  (Fig.  170,  A,  a)  in  the  form  of  a  ring,  the 
annulus,  and  a  part  may  also  hang  in  the  form  of  ragged  fila- 
ments from  the  edge  of  the  pileus.  If  a  volva  is  formed,  this 
is  also  ruptured  by  the  elongation  of  the  stipe,  forming  a  cup  at 
the  base  of  the  stipe,  and  usually  portions  also  remain  attached  to 
the  top  of  the  pileus  as  scales  and  patches  (Fig.  170,  A,  D). 
The  structure  of  the  mushroom  is  very  simple.  The  stipe  con- 
sists of  a  mass  of  nearly  parallel  hyphae  that  show  little  modi- 
fication save  at  the  surface  of  the  stem,  where  they  are  sometimes 
more  compactly  arranged  (Fig.  171,  A,  B).  By  cutting  across 
the  gills,  so  that  we  can  look  into  the  ends  of  them,  it  will  be 
seen  that  the  hyphae  extend  down  the  center  of  the  gills  and  also 
continually  radiate  out  on  either  side,  forming  a  compact  layer 
of  rather  elongated  cells  on  the  surface  of  the  gills,  known  as  the 
hymenium  (Fig.  171,  C).  A  magnified  view  of  a  portion  of  this 
hymenium  shows  that  it  is  composed  of  paraphyses  and  basidia 
(Fig.  171,  D}.  These  so-called  paraphyses  are  potential  basidia 
and  continue  to  develop  as  such  during  the  life  of  the  mushroom. 
The  basidia  are  not  divided  as  in  the  smuts  and  rusts,  but  the 
spores  are  formed  in  the  same  manner  at  the  end  of  two  or  four 
small  tubes  that  grow  out  from  the  apex  of  the  basidia.  It 
should  be  stated  that  a  series  of  intermediate  forms  exist  that 
connect  the  divided  basidia  with  the  present  form.  The  spores 
are  mature  and  begin  to  drop  off  as  soon  as  the  pileus  opens, 
when  they  are  scattered  byj:he  wind  and  develop  under  favorable 
conditions  into  a  new  mycelium.  If  the  pileus  of  a  freshly 
opened  mushroom  is  placed  on  a  sheet  of  gray  paper  and  tightly 
covered  with  a  dish,  so  as  to  exclude  all  air  currents,  the  spores 
will  fall  directly  upon  the  paper  and  in  a  few  hours  form  an 
exact  copy  or  a  spore  print  of  the  gill  arrangement.  There 


DEVELOPMENT   OF   PLANTS 


263 


a  sereveral  very  widely  distributed  and  familiar  families  of  the 
Agaricales,  distinguished  by  the  arrangement  and  distribution  of 
the  hymenium. 

A.    Thelephoraceae. — These  fungi  form  membranous,  leathery 


FIG.  171.  Structure  of  a  mushroom:  A  and  B,  cross  and  longitudinal 
sections  of  a  portion  of  the  stipe,  showing  the  character  and  arrangement 
of  the  hyphae  that  make  up  the  mushroom.  C,  tangential  view  of  the  gills 
— p,  pileus;  h,  hymenium  appearing  as  a  dark  band  on  the  surface  of  the 
gills.  D,  a  portion  of  the  hymenium  enlarged — &,  basidia;  pa,  paraphyses; 
s,  basidiospores. 

or  woody  incrustations  or  shell-like  structures  or  branching 
bodies  on  soil  or  wood  (Eig.  172,  A,  B).  The  hymenium  form- 
a  smooth  or  slightly  wrinkled  surface  on  the  under  side  or  exs 
posed  surface  of  the  fungus. 


264 


FAMILIES   OF   AGARICALES 


B.  Clavariaceae  or  Coral  Fungi. — This  family  includes  the  fairy 
clubs  and  coral-like  fleshy  masses  of  various  colors  (Fig.  172,  C). 
The  hymenium  covers  the  branches. 

C.  Hydnaceae  or  Prickly  Fungi. — These  fungi  form  masses  of 
widely  various  forms,  but  provided  with  spine-like  outgrowths 
upon  which  the  hymenium  is  developed  (Fig.  172,  D). 


FIG.  172.  Forms  of  the  Agaricales:  A,  Stereum,  top  view.  B,  underside, 
showing  the  characteristic  smooth  hymenial  surface  of  the  Telephoraceae. 

C,  a  coral  fungus,  Clavaria,  with  hymenium  confined  to  the  tips  of  the  branches. 

D,  a  prickly  fungus,  Hydnum,  with  the  hymenium  on  spine-like  projections. 

D.  Polyporaceae  or  Pore  Fungi. — This  group  includes  largely 
leathery,  woody  or  corky  fungi  in  which  the  hymenium  is  de- 
veloped on  the  surface  of  pores.  They  contain  some  of  the  most 
destructive  of  the  timber-destroying  fungi.  Many  of  the  genera 
of  this  family  form  shelf-like  outgrowths  on  trees  and  are  there- 
fore known  as  bracket  fungi  (Fig.  173,  B-D).  Professor  Duller 
estimated  that  a  single  pore  in  one  species  of  this  group  which  he 
found  growing  upon  a  decaying  elm,  produced  no  less  than 
1,700,000  spores  and  that  the  entire  bracket,  about  250  sq.  cm. 
in  area,  formed  over  eleven  billion  spores.  The  combined  annual 
output  of  the  ten  brackets  growing  upon  this  tree  exceeded  by 
fifty  times  the  population  of  the  globe.  This  enormous  spore 


DEVELOPMENT  OF  PLANTS 


265 


production  is  characteristic  of  nearly  all  of  the  Agaricales  and  it 
is  not  surprising  that  a  bit  of  dead  wood  or  piece  of  lumber  can 
not  be  exposed  without  being  infested  with  the  spores. 

E.  Boletaceae  or  Fleshy  Pore  Fungi. — The  members  of  this 
family  are  generally  characterized  by  a  stalk  and  a  pileus  which 


•i 


Urn 

m 


FIG.  173.  Pore  forms  of  the  Agaricales:  A,  Boletus,  showing  fleshy  pore- 
bearing  layer,  p.  B,  top  view  of  a  woody,  bracket  form,  Elfmngia.  The 
concentric  lines  represent  the  annual  growth.  C,  section  of  a  similar  form, 
showing  three  layers  of  pores  that  represent  three  years'  growth.  D,  en- 
larged view  of  under  surface,  showing  one  of  the  pores  with  hymenial  layer. 

bears  pores,  the  latter  being  easily  separable  as  a  layer  from  the 
pileus  (Fig.  173,  A). 

F.  Agaricaceae  or  Gill  Fungi. — These  fungi  more  commonly 
assume  an  umbrella  form,  although  some  of  them  are  of  the 
shelving  bracket  type  (Fig.  170).  The  hymenium  is  arranged 
on  the  surface  of  gills  or  plates.  Many  of  them  are  highly 
prized  for  the  table,  though  they  contain  comparatively  little 
nourishment,  and  must  be  regarded  as  relishes  rather  than  foods. 
While  the  majority  of  the  5,000  species  of  the  family  are  edible, 
some  of  them  contain  the  most  deadly  poisons.  No  rule  can  be 
given  that  will  enable  the  collector  to  separate  these  fungi  into 
the  two  mythical  groups  of  poisonous  toadstools  and  edible 
mushrooms.  Each  species  must  be  known  individually  before  it 
is  safe  to  use  them  for  food.  All  forms  with  a  volva  should  be 
most  carefully  identified,  because  this  is  a  feature  of  the  deadly 
amanitas  (Fig.  1 70) ,  which  are  among  the  most  poisonous  plants 
known. 
18 


266      CHARACTERISTICS   OF  THE   PUFF   BALLS 

The  remaining  orders  of  Basidiomycetes  are  characterized  by 
the  concealment  of  the  basidia  in  cavities  and  are  for  this  reason 
collectively  known  as  the  Gasteromycetes,  meaning  stomach 
fungi.  Among  the  more  common  orders  may  be  mentioned: 

98.  Order  d.     Lycoperdales  or  Puff  Balls. — These  familiar 


FIG.   174.     Cluster  of  common  puff  balls,  Lycoperdon.     At  left  three  older 
ones  have  opened,  permitting  discharge  of  basidiospores. 

fungi  are  developed,  as  in  the  Agaricales,  on  strands  of  the 
mycelium,  which  often  form  extensive  net-like  threads  in  rot- 
ten stumps,  logs,  sawdust  and  humus  (Fig.  174).  The  puff  balls 
vary  in  size  from  a  pea  to  over  a  foot  in  diameter.  When  young, 
they  consist  of  white  cheesy  masses  of  hyphae  which  form  in 


FIG.  175.  Diagram  of  a  section  of  one  of  the  puff  balls,  showing  the  thick 
skin  of  periderm  and  the  irregular  cavities  which  are  lined  with  basidia.  At 
the  base  the  larger,  sterile  cavities  of  the  stipe  are  shown. 

the  interior  of  the  puffball  a  series  of  irregular  cavities  lined  with 
basidia  and  on  the  exterior,  a  rather  firm  skin  or  periderm  (Fig. 
175).  At  maturity,  the  inner  hyphae  break  up,  leaving  only  a 


DEVELOPMENT   OF   PLANTS 


267 


dusty  mass  of  spores  and  in  some  cases  firmer  hyphae,  the  capil- 
litium.  The  skin  ruptures  in  various  ways,  often  by  one  or 
more  pores  at  the  top,  and  the  least  touch  now  causes  the  spores 
to  sift  out  in  smoke-like  puffs,  hence  the  popular  name  of  puff- 
balls.  In  the  earth  stars  (Fig.  176)  the  outer  layer  of  the  peri- 


FIG.  176.  One  of  the  puff  balls  popularly  known  as  "earth  stars,"  show- 
ing the  outer  periderm  splitting  into  star-like  sections  and  the  inner  peri- 
derm  opening  by  a  pore. 

derm  splits  into  rather  regular  star-like  segments  or  valves  which 
are  hygroscopic.  In  damp  weather  these  valves  roll  back,  in 
some  species  to  such  an  extent  as  to  lift  the  puffball  from  the 
ground,  when  it  may  be  set  rolling  by  the  wind  and  thus  bring 
about  a  better  discharge  of  the  spores. 


FIG.  177.  A  common  bird's-nest-fungus,  Crucibulum:  A,  habit  of  fungus 
on  branch  of  hickory.  B,  one  of  the  cups  in  section,  showing  the  tough  hyphae 
surrounding  the  spore-bearing  cavities. 

99.  Order  e.  Nidulariales  or  Bird's-Nest  Fungi. — These 
minute  and  curious  fungi  may  be  found  growing  upon  twigs  or 
upon  bare  ground  in  old  fields,  or  upon  dried  dung  (Fig.  177,  A}. 


268 


DEVELOPMENT   OF   PHALLALES 


They  differ  chiefly  from  the  puffballs  in  that  the  spore-bearing 
cavities  are  surrounded  by  tougher  hyphae.  Consequently, 
when  the  periderm  of  these  little  cup-shaped  bodies  opens,  these 
tougher  parts  appear  as  minute  eggs  in  a  nest  (Fig.  177,  B). 

100.  Order  f.  Phallales  or  Stink  Horns. — These  fungi  first 
appear  as  egg-like  structures  on  rather  coarse  strands  of  the 
mycelium  which  traverse  decaying  vegetation.  These  bodies 
consist  of  a  white  skin-like  periderm  which  encloses  a  stipe  and 


FIG.  178.  A  common  form  of  the  Phallales,  Phallus:  A,  the  so-called  egg- 
stage  which  in  B,  sectional  view,  is  seen  to  consist  of  an  outer  periderm,  p, 
and  within  is  a  central  stipe,  s,  capped  with  a  pileus.  The  spore-bearing 
cavities  of  the  pileus  are  shaded.  C,  stipe  and  pileus  have  emerged  from  the 
periderm,  which  forms  an  irregular  sac  about  the  base  of  the  stipe.  D,  later 
stage  after  the  spore-bearing  layer  has  dissolved,  revealing  the  cavities  on 
surface  of  pileus. 

pileus  (Fig.  178,  A,  B).  The  basidia  are  formed  in  honeycomb- 
like  cavities  on  the  outer  surface  of  the  pileus.  As  soon  as  the 
spores  are  matured  the  stipe  quickly  elongates,  rupturing  the 
periderm  and  lifting  the  pileus  into  the  air  (Fig.  178,  C).  In 
structure  and  coloration,  the  Phallales  are  among  the  most  attrac- 
tive of  the  Basidiomycetes,  but  the  majority  of  the  forms  are 
regarded  with  aversion,  since  the  spore-bearing  layer  melts 
down  into  a  slimy  mass  which  emits  a  carrion-like  stench. 


CHAPTER   VII 
DIVISION    II.     BRYOPHYTA.     THE   LIVERWORTS   AND   MOSSES 

101.  Adaptive  Features  of  the  Bryophyta. — We  have  now 
reached  the  point  in  the  evolution  of  the  plant  where  it  has  left 
the  aquatic  condition  of  its  algal  ancestors  and  become  largely 
terrestrial.  As  has  been  stated,  the  fungi  do  not  enter  into  the 
scheme  of  evolution  of  higher  types.  They  are  looked  upon 
as  a  side  line,  derived  doubtless  from  various  groups  of  the  algae, 
that  have  undergone  a  peculiar  series  of  variation  owing  to  their 
parasitic  and  saprophytic  habits  and  that  are  in  no  way  connected 
with  higher  plants.  In  the  Bryophyta  we  take  up  again  the  line 
of  ascent.  Naturally  we  would  expect  that  a  change  from  the 
aquatic  to  terrestrial  conditions  would  be  attended  with  profound 
changes,  tending  to  adapt  the  plants  to  the  terrestrial  conditions. 
The  results  of  the  stimulus  of  the  new  surroundings  are  seen  in 
the  production  of  hair-like  outgrowths,  rhizoids,  that  anchor  the 
plant  to  the  substratum  and  bring  it  into  proper  relation  with  the 
soil.  So  long  as  the  plant  lived  in  the  water  any  of  its  cells  might 
serve  as  absorbing  organs,  but  it  is  evident  that  the  provision 
must  now  be  made  for  procuring  the  crude  materials  which 
are  no  longer  in  direct  contact  with  the  plant.  The  exposure  of 
the  plant  to  the  atmosphere  results  also  in  the  formation  of  a 
cuticle  and  of  mucilage-producing  cells  which  tend  to  retain  the 
moisture  and  so  adapt  the  plant  to  its  drier  environment.  It 
will  also  be  noted  that  modifications  are  beginning  to  appear  in 
the  arrangement  of  the  chlorophyll-bearing  cells,  with  the  result 
that  they  are  sometimes  distributed  in  the  same  favorable  posi- 
tions for  photosynthesis,  as  you  have  noted  in  the  higher  plants. 
However,  the  Bryophyta  are  only  imperfectly  adapted  to  ter- 
restrial conditions  and  the  entire  plant  body  often  assists  in  the 
absorption  of  moisture  and  mineral  substances.  For  this  reason 
most  of  them  are  moisture  and  shade  plants,  some  indeed  being 
still  aquatic,  and  they  remain  rather  small  and  inconspicuous, 

269 


270      CHARACTERISTICS   OF  THE   BRYOPHYTA 

owing  to  the  fact  that  they  have  not  as  yet  varied  and  produced 
tissues  that  can  supply  them  with  an  adequate  amount  of  mois- 
ture and  protect  them  against  the  climatic  changes.  Owing  to 
their  habit  of  growing  together  in  colonies,  they  often  become 
conspicuous  and  cover  the  bare  earth,  logs  and  tree  trunks 
with  swards  and  mats  that  constitute  one  of  the  most  attractive 
features  of  the  forest  vegetation,  and  in  northern  regions  they 
often  form  the  most  characteristic  feature  of  the  vegetation 
over  large  districts. 

1 02.  General  Characteristics  of  the  Bryophyta. — Certain 
groups  of  the  Bryophyta  are  characterized  by  thalloid  plant 
bodies,  i.  e.,  without  distinction  of  stem,  root  or  leaf,  while  many 
have  developed  into  creeping  or  erect  plants  with  leaf -like  organs. 
All  are  distinguished  by  a  marked  localization  of  the  growing 
zone,  the  elongation  of  the  plant  being  due  to  the  repeated  divi- 
sions of  a  single  apical  cell.  The  reproduction  of  the  Bryophyta 
shows  such  marked  departures  that  we  realize  that  a  wide  gulf 
separates  even  the  simplest  of  them  from  the  Thallophyta. 
Notable  among  these  departures  is  the  absence  of  zoospores. 
Only  in  a  single  genus  are  nonciliated  bodies  formed  and  dis- 
charged from  certain  cells  as  in  the  case  of  zoospores.  In  this 
connection  it  is  worth  your  time  to  look  back  and  note  the 
trend  of  plant  life.  The  lowest  forms  of  the  green  algae  were 
essentially  motile.  Then  the  stationary  condition  arose  and 
the  motile  state  represented,  by  the  zoospores  became  less  and 
less  conspicuous.  In  the  bryophytes  we  perhaps  have  in  the 
spores,  referred  to  above,  an  interesting  example  of  the  last 
trace  of  the  motile  phase  of  plant  life.  The  suppression  by  the 
sexual  plant  of  the  spore  method  of  reproduction  is  due  to  several 
causes.  They  are  able  to  increase  their  numbers  in  a  vegetative 
manner.  Special  branches  or  ordinary  shoots  become  detached 
from  the  parent  plant  by  decay  of  the  older  parts  or  by  other 
means  and  develop  into  new  plants.  Buds,  called  gemmae, 
consisting  of  one  or  more  cells,  are  also  very  commonly  formed 
on  various  parts  of  the  plant  and  becoming  detached  grow  into 
new  plants  (Fig.  186).  Possibly  also  the  higher  organization  of 
the  Bryophyta,  which  renders  them  more  capable  of  meeting 


DEVELOPMENT   OF   PLANTS  271 

conditions  that  would  be  fatal  to  the  lower  forms,  tended  to 
make  them  independent  of  spore  reproduction.  But  especially 
has  this  change  been  brought  about  by  the  fact  that  the  gameto- 
spore  in  germinating  develops  an  asexual  plant  or  generation  with 
enormous  possibilities  in  the  way  of  spore  production,  so  that  the 
increase  and  distribution  of  individuals  is  now  fully  provided  for 
by  the  asexual  plant  and  consequently  largely  removed  from  the 
sexual  plant.  This  shifting  of  the  responsibility  for  the  increase 
in  number  of  individuals  from  the  sexual  to  the  asexual  generation 
is  one  of  the  most  significant  departures  in  the  evolution  of  plants. 
We  will  be  interested  to  note  how  this  change  came  about  and 
how  it  steadily  gains  in  importance  in  the  remaining  divisions. 

The  sexual  reproduction  of  the  bryophytes  reveals  a  series  of 
variations  that  are  only  remotely  suggestive  of  the  algae,  but  that 
indicate  very  clearly  relationship  with  higher  plants.  These 
features  will  be  considered  in  the  discussion  of  the  various  groups. 
There  are  two  classes  of  Bryophyta:  A,  The  Hepaticae  or  Liver- 
worts; B,  the  Musci  or  Mosses. 

Class  A.     Hepaticae  or  Liverworts 

103.  Classification  of  the  Liverworts. — While  these  plants  are 
the  simplest  and  least  conspicuous  of  the  Bryophyta,  they  are  the 
most  interesting,  because  among  them  will  be  found  reminders  of 
the  algae  and  at  the  same  time  they  present  many  features  that 
are  suggestive  of  the  mosses  and  ferns.     They  occur  in  very 
moist  places  and  in  deep  woods,  on  rotting  logs  and  moist  shady 
banks.     A  few  are  aquatic,  floating  upon  still  ponds  and  streams, 
and  some  have  become  adapted  to  dry  conditions.     There  are 
three   orders:    (a)    Marchantiales   or   Thallose   Liverworts;    (b) 
Jungermaniales    or    Leafy    Liverworts;    (c)    Anthocerotales    or 
Horned  Liverworts. 

104.  Order   a.     Marchantiales   or  Thallose   Hepatics. — The 
liverworts  of  this  order  are  characterized  by  flat,  prostrate  and 
rather  fleshy  bodies  which  usually  grow  upon  the  earth,  to  which 
they  are  attached  by  numerous  rhizoids.     Owing  to  the  frequent 
division  of  the  apical  cell  of  the  thallus  into  two  equal  parts,  there 
result  two  growing  points  which  thus  produce  the  equal  forking 


272  STRUCTURE  OF  RICCIOCARPUS 

or  dichotomous  branching  of  the  thallus,  so  characteristic  of 
these  plants  (Fig.  179,  A,  B).  The  appearance  of  many  of  these 
hepatics  is  suggestive  of  the  algae.  Especially  is  this  true  of  the 
aquatic  Ricciocarpus  and  Riccia. 

(a)  Structure  of  Ricciocarpus. — An  examination  of  the  struc- 
ture of  one  of  these  will  show,  however,  that  extensive  changes 
have  been  induced  in  even  the  simplest  forms.  The  new  stimuli 
to  which  the  terrestrial  conditions  expose  them  cause  a  remark- 
able series  of  transformations  in  the  cells  that  are  cut  off  from  the 


c 

FIG.  179.  Forms  of  semiaquatic  Marchantiales :  A,  Riccia,  showing  the 
dichotomous  branching  of  the  thallus.  B,  Ricciocarpus.  The  sexual  or- 
gans are  concealed  in  furrows  that  appear  as  radiating  lines  in  the  center 
of  the  branches.  C,  diagram  of  a  cross-section  of  a  branch,  showing  the 
male  gametangia  in  the  bottom  of  one  of  the  furrows. 

apical  cell.  The  upper  cells  of  the  thallus,  as  soon  as  they  are 
formed  at  the  growing  point,  are  exposed  directly  to  the  air  and 
light,  and  they  develop  chlorophyll  and  grow  up  into  vertical 
rows  or  plates  just  as  you  have  already  noticed  in  the  palisade 
chlorenchyma  of  the  leaf  (Fig.  180,  A).  At  an  early  period  these 
rows  of  green  cells  become  separated  so  that  air  spaces  arise 
between  them,  and  thus  the  chlorophyll-bearing  cells  are  brought 
into  direct  contact  with  the  atmosphere  and  enabled  to  carry  on 
photosynthesis  to  the  best  advantage.  The  terminal  cells  of 
these  rows  enlarge  considerably  and  form  a  rudimentary  epider- 
mis. It  is  also  interesting  to  note  that  the  direct  contact  of  the 
cells  with  the  atmosphere  results  for  the  first  time  in  the  forma- 


DEVELOPMENT   OF   PLANTS 


273 


tion  of  a  cuticle.  In  most  species  the  air  spaces  arising  among  the 
upper  cells  of  the  thallus  increase  greatly  in  size  and  the  epidermal 
cells  by  vertical  divisions  keep  pace  with  this  enlargement  and 
thus  arch  over  the  air  cavities,  forming,  however,  a  small  opening 
in  the  epidermal  layer  that  permits  a  free  circulation  of  the  air 
(Fig.  1 80,  B).  These  openings  are  suggestive  of  stomata  and 
function  in  the  same  way,  though  they  have  originated  in  an 
entirely  different  manner.  The  lower  cells  of  the  thallus  are 
compact  and  doubtless  serve  as  storage  cells,  being  nearly  or 
quite  destitute  of  chlorophyll.  The  stimulus  of  the  soil  causes 


FIG.  1 80.  Structure  of  the  thallus  of  Ricciocarpus:  A,  section  of  the 
thallus,  showing  the  apical  cell,  x,  forming  cells  that  by  further  division  de- 
velop into  plates  of  cells  separated  by  air  spaces,  j.  At  the  left  the  plates 
thus  formed  are  seen  curving  over  the  apical  cell.  B,  an  older  portion  of 
the  upper  part  of  the  thallus.  The  air  spaces,  j,  are  greatly  enlarged  and 
the  upper  cells  of  the  vertical  plates  have  divided,  arching  over  the  air  spaces 
but  leaving  small  openings  which  permit  the  entrance  of  air  for  photosyn- 
thesis.—I.  D.  Cardiff. 

the  lower  epidermal  cells  of  the  thallus  to  form  numerous  smooth 
or  pitted  rhizoids  that  are  suggestive  of  the  root-hairs  of  the  higher 
plants,  anchoring  the  plant  to  the  substratum  and  assisting  in  the 
absorption  of  the  earth  substances.  Plates  of  cells  similar  in 
origin  to  those  occurring  on  the  upper  surface  are  also  found 
on  the  under  side  of  the  thallus  (Fig.  180,  A).  These  plates 
curve  up  around  the  growing  point  and  doubtless  protect  it 
against  drought,  in  which  work  they  are  assisted  by  the  mucilage 


274 


REPRODUCTION  OF   RICCIOCARPUS 


cells  or  glands.  The  development  of  these  glandular  cells  on  the 
plates,  as  well  as  on  the  other  parts  of  the  thallus,  adapt  the 
plants  to  the  drier  terrestrial  conditions  to  which  they  are 
exposed,  as  was  the  case  with  the  mucilaginous  walls  of  the 
Schizomycetes  and  Zygnematales. 

(b)  Sexual  Reproduction  of  Ricciocarpus. — The  gametes  are 
produced  in  more  complex  gametangia  than  we  have  as  yet  seen. 
These  organs  are  developed  upon  the  upper  surface  of  the  thallus 


B 


FIG.  181.  The  origin  and  structure  of  the  male  gametangia  or  antheridia: 
A,  section  of  the  thallus,  showing  the  apical  cell,  x,  and  the  early  stages, 
a,  &,  in  the  development  of  the  antheridia.  B,  older  antheridium  with  cells 
dividing  vertically.  C,  later  stage  in  which  the  wall  cells  are  differentiated. 
D,  mature  antheridium  of  Marchantia,  showing  the  numerous  cells  that 
develop  the  male  gametes  and  the  wall  cells,  w.  E,  greatly  enlarged  view 
of  a  male  gamete  after  discharge  from  the  antheridium. 

and  in  some  species  appear  as  lines  radiating  from  the  center  of 
the  plant.  The  male  gametarigium  originates  from  one  of  the 
superficial  cells  of  the  thallus,  which  at  first  continues  to  divide 
transversely  after  the  manner  of  the  vertical  plates  of  chlorophyll- 
bearing  cells  (Fig.  181,  A).  The  cells  of  these  vertical  plates 
soon  begin  to  divide  vertically  and  thus  form  an  elliptical  mass 
of  cells  (Fig.  181,  Bj  C).  As  this  growth  goes  on  the  outer  or 
wall  cells  become  larger  than  the  others  and  generally  develop 
chlorophyll,  while  the  inner  cells  divide  repeatedly  and  become 


DEVELOPMENT   OF   PLANTS  275 

very  numerous  with  dense  granular  contents  (Fig.  1 8 1 ,  D) .  Each 
of  the  latter  cells  produces  a  single  male  gamete  that  is  motile  by 
means  of  two  cilia  (Fig.  181,  E).  This  complex  gametangium  is 
suggestive  of  the  structure  we  have  seen  in  Ectocarpus  (Fig.  119, 
B) .  The  male  gametangium  is  commonly  called  the  antheridium, 
and  the  male  gametes  are  often  termed  sperms  or  antherozoids. 
The  gametes  are  discharged  from  the  antheridium  owing  to  the 
absorption  of  water  by  the  walls  of  the  cells  in  the  upper  portion  of 
the  antheridium  which  become  mucilaginous  at  maturity  and  ex- 
panding, rupture  this  region  of  the  antheridium,  thus  permitting 
the  discharge  of  its  contents.  The  walls  of  the  gamete  producing 
cells  also  become  mucilaginous  and  by  the  absorption  of  moisture 
they  may  assist  in  the  extrusion  of  the  gametes.  The  pressure  of 
the  surrounding  tissues  upon  the  antheridium  is  also  an  important 
factor  in  this  discharge.  The  mucilaginous  mass  that  is  extruded 
from  the  antheridium  in  this  manner,  rapidly  dissolves  in  the 
water  and  sets  free  the  male  gametes  or  sperms.  The  female 
gametangium  is  also  quite  different  from  the  single-celled  game- 
tangia  which  we  have  noticed  among  the  algae  and  fungi.  It 
originates  as  in  the  case  of  the  antheridium,  but  develops  into  a 
flask-shaped  body,  consisting  of  a  long  neck  of  several  rows  of 
cells  surrounding  a  central  row,  called  the  canal  cells,  and  an  en- 
larged basal  region  in  which  is  developed  the  female  gamete  (Fig. 
182).  The  female  gametangium  is  commonly  known  as  the 
archegonium  and  the  female  gamete  is  sometimes  called  the 
oosphere  or  egg.  The  female  gamete  presents  essentially  the 
same  characteristics  as  seen  in  many  of  the  algae,  as  Vaucheria, 
Oedogonium,  etc.,  but  it  is  surrounded  by  a  jacket  of  cell  which 
constitutes  the  basal  portion  of  the  archegonium  instead  of  being 
contained  in  a  single  cell.  Possibly  the  archegonium  has  come 
about  from  a  multicellular  gametangium  as  seen  in  the  brown 
algae,  owing  to  the  sterilization  of  all  but  one  of  the  cells.  Arche- 
gonia,  with  two  or  more  gamete-like  cells,  are  sometimes  found 
in  the  mosses  and  ferns  and  there  is  evidence  tending  to  show 
that  the  primitive  gametangia  contained  both  male  and  female 
gametes.  As  soon  as  the  female  gamete  is  formed,  the  apical  or 
lip  cells  of  the  neck  open  in  the  same  manner  as  noted  in  the 


276 


ARCHEGONIUM    OF   RICCIOCARPUS 


antheridium  and  the  canal  cells  become  mucilaginous,  thus 
forming  a  passageway  to  the  female  gamete  (Fig.  182,  F).  The 
male  gametes,  attracted,  it  is  supposed,  by  cane  sugar,  developed 
in  the  archegonium,  swim  down  the  canal  of  the  neck,  and  one 
unites  with  the  female  gamete,  as  shown  in  Fig.  182,  F,  where 


FIG.  182.  Development  of  the  female  gametangium  or  archegonium, 
A-D,  stages  in  the  development  of  the  archegonium  from  a  single  cell.  Et 
nearly  mature  archegonium  just  before  the  female  gamete  is  formed — en, 
canal  cells.  F,  lip  cells  have  opened  and  the  canal  cells  have  dissolved,  thus 
forming  a  passage  way  to  female  gamete.  A  male  gamete  has  entered  and 
is  seen  fusing  with  the  nucleus  of  the  large  female  gamete. — After  Garber. 

the  two  gametes  are  seen  fusing.  Note  that  mucilage  retained 
in  the  neck  protects  the  female  gamete  against  loss  of  water  and 
also  that  the  attenuated  form  of  the  male  is  of  advantage  in 
enabling  it  to  work  its  way  through  this  substance. 

(c)  Germination  of  the  Gametospore  and  Spore  Formation. — 
The  gametospore,  resulting  from  this  fusion,  becomes  surrounded 
by  a  cell  wall,  but  does  not  function  as  a  resting  spore  as  in  some 
of  the  algae.  It  germinates  at  once  and  by  repeated  divisions  of 


DEVELOPMENT   OF   PLANTS 


277 


its  nucleus  forms  a  globular  mass  of  cells,  the  capsule,  within  the 
archegonium  which  keeps  pace  with  its  growth  (Fig.  183,  A,  B). 
The  wall  cells  of  this  capsule  are  soon  distinguishable,  owing  to 
their  watery  contents  and  shape,  from  the  other  cells,  which  are 
rather  cubical  and  densely  granular.  The  growth  and  division 
of  the  granular  cells  continues  until  about  400  have  been  formed, 
when  they  round  off,  become  separated  from  one  another  and 
increase  greatly  in  size  (Fig.  183,  C).  These  large  cells,  called 


FIG.  183.  Germination  of  the  gametospore:  A,  basal  portion  of  an  arche- 
gonium, showing  the  germinating  gametospore  in  the  four-cell  stage.  B, 
later  growth,  forming  a  capsule  with  wall  cells,  w,  which  inclose  large  gran- 
ular cells.  C,  a  portion  of  the  capsule,  showing  spore  mother  cells  rounding 
off  and  floating  in  the  fluid  of  the  capsule.  D,  the  mother  cells  dividing 
and  forming  four  spores  each. — H.  O.  Hanson. 

spore  mother  cells,  form  four  spores  each,  as  in  the  tetraspores 
of  the  red  algae  (Fig.  183,  D).  The  delicafe  walls  of  the  cap- 
sule break  down  as  soon  as  the  spores  are  matured,  leaving  them 


278  THE  SPORES  OF  RICCIOCARPUS 

free  in  the  archegonium,  and  later  they  are  set  free  by  the  decay 
of  the  latter  organ.  These  spores  in  some  forms  are  provided 
with  thick  walls  and  are,  therefore,  resting  spores  adapted  to 
carrying  the  plant  over  unfavorable  conditions  for  growth.  In 
other  cases  the  spores  have  thin  walls  and  germinate  at  once. 
The  advantage  of  the  transference  of  the  resting  stage  from  a 
single  gametospore  to  the  numerous  spores  derived  from  the 
gametospore  is  manifest. 

(d)  Germination  of  the  Spore. — The  germination  of  the  spore 
is  usually  indicated  by  the  formation  of  chlorophyll,  and  this  is 


FIG.  184.  Germination  of  the  spores:  A,  spore.  B,  first  division  of  the 
germ  tube.  C,  early  form  of  the  thallus,  due  to  the  formation  and  subse- 
quent division  of  the  apical  cell. 

followed  by  the  rupture  of  the  outer  spore  coat  and  the  protru- 
sion of  the  inner  as  a  delicate  papilla  or  germ  tube  (Fig.  184). 
Usually  from  this  tube  a  small,  hair-like  outgrowth  is  soon 
formed  which  penetrates  the  soil  as  the  first  rhizoid.  The  germ 
tube  continues  to  elongate  and  often  forms  a  chain  of  cells  by 
successive  transverse  divisions,  but  eventually  by  oblique  divi- 
sions an  apical  cell  is  formed  that  develops  the  characteristic 
thallus  (Fig.  184,  C).  Thus  we  arrive  again  at  the  starting  point 
in  the  life  history  of  these  simple  plants. 

(e)  Noteworthy  Departures  in  the  Life  History. — It  should  be 
stated  that  the  gametophyte  of  certain  species  of  the  Marchan- 


DEVELOPMENT   OF   PLANTS  279 

tiales  may  live  for  long  periods  without  producing  the  gametes. 
In  fact,  it  frequently  multiplies  by  means  of  buds  and  branches 
which  become  detached  and  grow  directly  into  new  plants.  How- 
ever, as  soon  as  conditions  are  favorable  the  sexual  organs  and 
gametes  will  appear.  There  are  several  features  in  this  history 
that  must  be  kept  clearly  in  mind.  In  the  first  place,  the 
gametospore  is  not  discharged  from  the  plant  as  in  the  case  of  the 
green  or  brown  algae,  but  remains  permanently  in  the  arche- 
gonium,  where  it  continues  to  be  nourished  by  the  plant  during 
its  germination  and  the  formation  of  its  spores.  Therefore,  it 
develops  essentially  as  a  parasite  upon  the  plant,  as  is  the  case 
among  the  red  algae.  This  retention  and  nourishment  of  the 
germinating  gametospore  in  the  plant  is  perhaps  the  most 
important  of  any  of  the  variations  that  appear  in  plant  life. 
Owing  to  this  relationship  the  gametospore  attains  a  larger  growth 
with  increased  power  of  spore  production.  In  Ricciocarpus  the 
capsule  is  about  500  times  as  large  as  the  gametospore.  In  this 
way  the  development  of  many  new  plants  is  made  possible  by  a 
single  fusion  of  gametes.  This  is  a  very  significant  feature  in 
terrestrial  plants,  since  the  fusion  of  the  gametes  is  effected  with 
more  difficulty,  owing  to  the  absence  of  aquatic  conditions. 
Without  doubt  this  change  was  induced  by  the  transference  of 
foods  from  the  sexual  to  the  asexual  plant  derived  from  the 
gametospore.  This  loss  of  food  gradually  prevented  the  sexual 
plant  from  producing  spores  or  other  bodies  designed  to  multiply 
its  numbers  and  we  will  finally  see  that  the  sole  work  of  the  sexual 
plant  is  limited  to  the  production  of  gametes.  Note  also  that 
the  gametospore  not  only  has  a  larger  growth  but  a  longer  life. 
This  is  of  the  utmost  importance  because  it  became  exposed  in 
this  way  to  a  new  series  of  stimuli  that  affected  it  profoundly  and 
that  resulted  in  the  evolution  of  the  higher  types  of  plants. 
Among  the  green  algae  the  actual  germination  of  the  gameto- 
spore is  limited  to  a  few  hours  at  the  most  and  this  is  effected  in 
the  water,  where  the  conditions  are  exceptionally  uniform.  The 
germination  of  the  gametospore  in  the  case  of  the  liverworts  is 
prolonged  over  several  weeks,  and  more  important  still  is  the 
fact  that  this  growth  occurs  on  the  land  where  it  is  exposed  to  a 


280  THE  TWO   PHASES   IN   PLANT   LIFE 

wide  range  of  stimuli  that  cause  it  to  vary  in  a  most  remarkable 
manner.  So  we  must  bear  in  mind  the  character  of  this  rather 
simple  growth  derived  from  the  gametospore  of  Ricciocarpus  and 
compare  it  with  the  variations  that  appear  in  the  succeeding 
forms. 

It  is  also  evident  that  the  fusion  of  the  gametes  in  the  simple 
liverworts  results  in  the  production  of  a  cell,  the  gametospore, 
that  is  radically  different  in  its  nature  and  possibilities  of  growth 
from  any  of  the  cells  of  the  parent  plant  or  liverwort.  This  was 
not  apparent  in  the  simpler  forms  of  the  algae,  where  the  gameto- 
spore produced  directly  a  plant  like  the  parent,  as  in  Vaucheria 
and  Fucus.  In  Spirogyra,  Ulothrix,  Oedogonium  and  Coleo- 
chaete,  we  saw  the  first  indication  of  the  real  nature  of  the  game- 
tospore. It  did  not  develop  into  a  plant  like  the  parent,  but 
produced  cells  or  zoospores.  The  reason  for  this  difference  in 
behavior  is  possibly  due  to  the  higher  organization  of  the  gameto- 
spore. In  the  lower  forms  its  first  divisions  result  in  the  forma- 
tion of  cells  that  are  of  the  same  nature  as  the  cells  of  the  parent 
plant,  but  in  higher  forms  its  composition  is  more  complex  and  as 
a  consequence  it  forms  a  number  of  cells  before  cells  like  those  of 
the  parent  plant  are  developed.  For  example,  in  the  Red  Algae, 
certain  of  the  fungi  and  in  Ricciocarpus  the  gametospore  gives  rise 
to  a  varying  number  of  cells  that  are  different  from  those  of  the 
parent  plant,  but  finally  spore  mother  cells  are  formed  from  which 
are  derived  spores  that  are  of  the  same  nature  as  those  of  the 
parent  plant  and  that  consequently  produce  new  plants  like  the 
parent.  As  has  been  stated  on  page  138,  one  difference  between 
the  cells  derived  from  the  gametospore  and  those  of  the  parent 
plant  is  to  be  found  in  the  number  of  chromosomes  which  they 
contain,  the  former  having  twice  as  many  chromosomes  as  the 
latter.  The  doubling  of  the  chromosomes  occurs  when  the  two 
gametes  unite  to  form  the  gametospore  and  this  number  is  re- 
tained during  the  germination  of  the  gametospore  until  the  divi- 
sion of  the  spore  mother  cells,  which  results  in  the  formation  of 
spores  with  only  one  half  the  number  of  chromosomes.  In  the 
lower  algae  this  reduction  must  occur  in  the  first  divisions  of  the 
germinating  gametospore,  which  therefore  corresponds  to  the 


DEVELOPMENTS  OF   PLANT 


281 


spore  mother  cell,  but  in  the  Red  Algae  and  Ricciocarpus  a  con- 
siderable growth  intervenes  before  the  spore  mother  cells  appear 
and  the  reduction  of  the  chromosomes  takes  place.  In  other 
words,  as  we  ascend  the  scale  of  plant  life,  the  formation  of  the 
spore  mother  cells  and  the  reduction  of  the  chromosomes  is  pre- 
ceded by  an  ever-increasing  growth  of  the  gametospore.  This 
postponement  in  the  formation  of  the  spore  mother  cells,  owing 
to  the  larger  and  larger  growth  of  the  gametospore,  will  steadily 
progress  in  the  following  studies. 

These  simple  liverworts,  like  Ricciocarpus,  show  very  clearly 
two  phases  or  generations  in  their  life  history.     The  thallose 


FIG.  185.  Diagram  of  the  life  history  of  Ricciocarpus.  The  upper  por- 
tion of  the  figure  represents  the  sexual  generation  and  the  lower  portion, 
the  asexual.  The  former  generation  begins  with  the  formation  of  the  spores, 
sp,  from  the  mother-cell  and  ends  with  the  formation  of  the  gametes,  g.  The 
asexual  generation  begins  with  the  gametospore,  gm,  and  ends  with  the  spore 
mother  cells,  sm. 

plant  is  the  gametophyte  or  sexual  generation  because  it  bears 
sexual  cells  or  gametes.  The  capsule  is  the  sporophyte  or  asex- 
ual generation  because  it  can  only  produce  spores.  The  gameto- 
phyte begins  with  the  spore  and  ends  with  the  formation  of  the 
gametes.  The  sporophyte  begins  with  the  gametospore  and  ends 
with  the  division  of  the  spore  mother  cell  (Fig.  185).  It  may 
appear  to  you  now  as  strange  to  regard  the  few  cells  of  the  cap- 
sule, the  majority  of  which  become  spore  mother  cells,  as  a  plant. 
But  we  shall  directly  see  this  microscopic  plant  assuming  larger 
proportions  as  a  result  of  its  better  nourishment  and  the  stimuli 
to  which  it  is  exposed.  It  will  form  a  larger  and  larger  number 
19 


282  STRUCTURE   OF   MARCHANTIA 

of  cells  and  the  spore  mother  cells  will  not  appear  until  late  in  its 
growth  and  they  will  be  confined  to  definite  regions  and  only  con- 
stitute a  small  part  of  it.  We  now  call  the  gametophyte  the  plant 
because  it  is  many  times  larger  than  the  sporophyte,  but  we  shall 
see  the  sporophyte  become  quite  as  conspicuous  as  the  gameto- 
phyte, as  in  the  mosses,  and  finally  it  will  become  much  larger, 
as  in  the  ferns.  Then  we  shall  call  the  sporophyte  the  plant. 

(/)  Structure  and  Life  History  of  Marchantia. — Let  us  now 
examine  a  higher  type  of  the  Marchantiales,  as  Marchantia,  and 
note  the  advances  that  have  been  made  over  simple  forms  like 
Ricciocarpus.  Marchantia  (Fig.  186)  is  of  common  occurrence 


FIG.  1 86.  FIG.  187. 

FIG.  1 86.  Thallus  of  Marchantia  bearing  three  cup-like  organs  that  con- 
tain buds  or  gemmae  and  also  four  erect  branches  that  contain  the  anther- 
idia  in  their  upper  surfaces  in  radiating  lines. 

FIG.  187.  Portion  of  the  surface  of  the  thallus  of  Marchantia  enlarged, 
showing  the  rhomboidal  air  chambers  and  air  pores. 

in  moist  places,  often  appearing  in  greenhouses  on  the  earth  of 
flower  pots,  and  it  also  forms  luxuriant  beds  on  the  damp  ground, 
especially  where  logs  and  brush  have  been  burnt.  The  thallus 
shows  the  same  general  features  that  we  have  noticed  in  Riccio- 
carpus, being  rather  fleshy  and  creeping  over  the  ground,  to  which 
it  is  attached  by  numerous  rhizoids.  The  simple  air  chambers 
noted  in  Ricciocarpus  are  much  enlarged  and  appear  as  diamond- 
shaped  or  rhomboidal  plates  on  the  surface  of  the  thallus  (Fig. 
187).  Thin  sections  across  the  thallus  show  that  these  air 
chambers  have  a  complex  structure  (Fig.  188).  They  originate 
in  the  upper  cells  of  the  thallus  and  as  they  enlarge  they  become 
covered  by  a  well-developed  epidermis  which  forms  a  chimney- 


DEVELOPMENT   OF   PLANTS  283 

like  pore  over  the  center  of  each  cavity.  From  the  bottom  of 
the  chamber  numerous  delicate  chlorophyll-bearing  cells  arise. 
This  arrangement  of  the  tissues  is  again  suggestive  of  the  chlor- 
enchyma  of  the  leaf  and  it  is  manifestly  protective  and  adapted 
to  photosynthesis.  The  structure  of  the  lower  cells  of  the  thallus 


FIG.  1 88.  Section  through  the  center  of  the  thallus  of  Marchantia,  show- 
ing one  of  the  air  chambers  and  chimney-like  pores  in  the  epidermis — ch, 
palisade-like  chlorenchyma  arising  from  bottom  of  air  chamber.  The  lower 
cells  of  the  thallus  are  nearly  colorless  and  filled  with  watery  solutions  or 
mucilage,  r,  rhizoids;  /,  leaf-like  plates  of  cells. 

and  the  distribution  of  the  rhizoids  and  ventral  plates  are  essen- 
tially as  in  Ricciocarpus . 

Marchantia,  as  in  many  of  the  liverworts  and  some  mosses, 
multiplies  extensively  by  means  of  buds  or  gemmae.  They  are 


FIG.  189.     Diagram  of  a  section  of  one  of  the  cups*shown  in  Fig.  186 — g, 
buds  or  gemmae  associated  with  small  glandular  cells. 


284  REPRODUCTION   OF   MARCHANTIA 

borne  in  cup-shaped  receptacles  and  consist  of  small  lense-shaped 
masses  of  cells  which  are  freed  from  the  thallus  by  the  swelling 
of  the  mucilage  secreted  by  glandular  hairs  growing  among  them 
(Fig.  189).  These  minute  bodies  grow  directly  into  new  plants 
and  so  serve  to  rapidly  multiply  the  plant. 

The  reproductive  organs  appear  upon  different  plants  which 


FIG.  190.  Thallus  of  Marchantia  bearing  several  erect  branches  with 
lobed  or  umbrella-like  tops.  The  archegonia  are  situated  on  the  underside 
of  the  umbrella,  between  the  lobes,  a,  the  early  appearance  of  a  branch, 
in  wfiich  condition  fertilization  is  effected. 

may  therefore  be  distinguished  as  antheridial  or  male  plants  and 
archegonial  or  female  plants  (Figs.  186.  190).  Such  a  distribu- 
tion of  the  sexual  organs  is  termed  dioecious,  meaning  in  two 
households,  whereas  Ricciocarpus  and  others  are  said  to  be  mon 
oecious,  because  the  sexual  organs  are  developed  upon  the  same 
plant.  The  antheridia  and  archegonia  are  developed,  as  in  Ric- 
ciocarpus, upon  the  upper  surface  of  the  thallus,  but  they  are 
confined  to  special  portions  of  it  which  appear  at  first  as  mush- 
room-shaped outgrowths  (Fig.  190,  a).  These  outgrowths,  how- 


DEVELOPMENT   OF   PLANTS 


285 


ever,  are  but  modified  branches  of  the  thallus  and  bear  the 
sexual  organs  upon  their  upper  surfaces  in  lines  radiating  from 
the  center  of  the  branches.  Fertilization  is  effected,  as  in  all 
forms,  by  means  of  dews  and  rains  which  flood  these  organs  with 
water.  An  examination  of  a  young  archegonial  branch  will  show 
how  admirably  it  is  constructed  to  ensure  fertilization.  The 
archegonia  radiating  in  lines  have  their  necks  strongly  curved 


FIG.  191.  FIG.  192. 

FIG.  191.  Section  of  an  archegonial  branch  similar  to  a,  Fig.  190 — ar, 
archegonia;  in,  involucre  or  curtain  that  hangs  down  on  either  side  of  the 
rows  of  archegonia;  ac,  air  chambers;  th,  thallus. 

FIG.  192.  Section  of  a  young  antheridial  branch:  an,  antheridia  sunken 
in  cavities  of  the  branch,  which  is  also  provided  with  air  chambers  similar 
to  those  of  the  normal  thallus.  Some  of  the  antheridia  have  discharged 
their  gametes,  as  at  x. 

upwards  and  away  from  the  center  of  the  branch.  These  lines 
of  archegonia  are  separated  by  ridges  which  thus  act  as  a  water- 
shed, deflecting  into  the  upturned  necks  any  water  containing 
gametes  that  may  chance  to  fall  upon  them.  After  fertilization 
these  mushroom-shaped  branches  elongate,  raising  the  sexual 
organs  up  into  the  air,  where  the  antheridial  branch  assumes  the 
form  of  a  lobed  disc,  while  the  archegonial  branch  terminates  in 
an  umbrella-like  structure  (Figs.  186,  190).  Owing  to  the  ex- 
tended growth  of  this  latter  branch,  the  archegonia  come  to  lie 
on  the  under  side  of  the  structure  between  the  finger-like  out- 
growths, where  they  are  completely  hidden  and  protected  by 
fringed  curtains,  the  involucre,  that  hang  down  from  the  fingers 


286 


SPOROPHYTE  OF  MARCHANTIA 


(Figs.  191,  195,  A).  The  antheridia  retain  their  original  position 
upon  the  branch,  where  they  appear  in  cavities,  as  shown  in 
Fig.  192. 

The  most  important  and  significant  departure  in  Marchantia  is 
seen  in  the  germination  of  the  gametospore  which  divides  into  an 
inner  and  outer  cell  as  in  Ricciocarpus  (Fig.  193,  B).  The  inner 


FIG.  193.  Germination  of  the  gametospore:  A,  section  of  a  mature  ar- 
chegonium  with  canal  cells  dissolved,  thus  forming  a  passageway  to  the 
large  female  gamete,  g.  B,  sectional  view  of  base  of  archegonium,  show- 
ing the  germinating  gametospore  in  two-cell  stage.  The  perianth,  p,  is 
seen  growing  up  about  the  archegonium.  C,  later  stage  in  growth  of  the 
gametospore.  The  lower  cell  shown  in  B  is  forming  stalk  cells,  while  the 
outer  cell  has  produced  densely  granular  cells  that  will  later  by  further  di- 
vision form  spore  mother  cells  and  elaters. 

cell,  however,  by  a  series  of  divisions,  forms  a  rudimentary  stalk 
and  the  lower  part  of  it,  known  as  the  foot,  comes  into  close 
contact  with  the  tissues  of  the  plant  from  which  it  absorbs  nour- 
ishment (Figs.  193,  194).  The  outer  cell  divides,  forming  a 
spherical  mass  of  cells  that  resembles  the  capsule  of  Ricciocarpus. 
Some  of  these  cells  of  the  capsule  develop  as  spore  mother  cells, 


DEVELOPMENT   OF   PLANTS 


287 


while  others,  known  as  elaters,  elongate  greatly  and  serve  to  con- 
duct the  foods  absorbed  by  the  foot  to  the  spore  mother  cells,  and 
finally  they  become  spirally  thickened  (Fig.  194,  C).  These 
elaters  arise  through  the  sterilization  of  certain  of  the  spore- 


FIG.  194.  Structure  of  the  nearly  mature  sporophyte:  A,  continuation 
of  the  growth  in  Fig.  193,  C,  showing  the  formation  of  a  foot,  /,  stalk,  and 
capsules,  c,  which  contain  elongated  dark  cells,  the  elaters,  and  the  spores, 
The  archegonium  has  been  ruptured  by  the  elongation  of  the  stalk  and  is 
not  shown  in  the  figure,  p,  perianth.  B,  enlargement  of  the  base  of  A . 
showing  the  attachment  of  the  foot  (indicated  by  darker  lines)  to  the  tissues 
of  the  antheridial  stalk.  C,  an  elater  and  spores. 


FIG.  195.  Archegonial  branch  with  mature  sporophytes:  A,  branch  with 
the  dark  capsules  of  several  sporophytes  projecting  beyond  the  curtains  of 
the  involucre.  B,  diagram  of  a  branch  as  seen  in  section.  On  the  right 
one  of  the  capsules  of  a  sporophyte  ruptured,  exposing  the  elaters  and  spores- 
On  the  left  the  curtain-like  involucre  only  is  shown. 


288  SPOROPHYTE   OF   MARCHANTIA 

producing  cells.  This  is  an  important  departure  from  Riccio- 
carpus  and  we  see  here  the  beginning  of  the  tendency  towards 
the  sterilization  of  sporogeneous  cells  that  becomes  more  and 
more  pronounced  in  higher  forms  and  that 
plays  an  important  part  in  the  evolution  of 
plant  life.  During  the  germination  and  growth 
of  the  gametospore,  a  delicate  membrane  (the 
perianth)  grows  up  about  the  archegonium 
and  doubtless  assists  the  involucre  in  pro- 
tecting it  against  drying  winds  ( Fig.  193, 
p}.  When  the  spores  are  mature,  the  cells 
of  the  stalk  elongate,  rupture  the  archego- 
nium and  push  the  capsule  beyond  the  cur- 
tains of  the  perianth  and  involucre,  so  that 
it  is  exposed  to  the  air  (Fig.  195).  The  cap- 
sule now  ruptures,  exposing  the  spores  and 
elaters  to  the  air.  The  elaters,  a  word  mean- 
ing lifters,  are  very  hygroscopic;  they  coil  and 
uncoil  with  the  least  change  in  the  humidity 
of  the  air  and  thus  doubtless  assist  in  the  grad- 
ual exposure  of  the  spores  to  the  air  currents. 
FIG.  196.  One  of  The  spores  germinate  as  in  the  lower  liver- 
simpler  thalloidjun-  worts>  Thus  we  see  that  the  sporophyte, 
germaniales,  Pallavi-  ....  , 

cinia,   showing  the    consisting  of  a  foot,  stem  and  capsule,  is  more 

rhizoidal  growth  on  complex  and  larger  than  in  Ricciocarpus. 
the  under  surface  of  This  is  doubtless  due  to  the  better  nourish- 

the    thallus    and    a  i_«  i     •,  •  i.*.     r  j.t_     J 

ment  which  it  receives  as  a  result  01  the  de- 
mature    sporophyte 

arising  from  a  cup-  velopment  of  a  more  efficient  absorbing  organ, 
like  perianth  which  the  foot. 

is  surrounded  at  its  Order  b      Jungermaniales  or  Leafy 

base  with  mvolu- 

crate  leaves.— H.  O.    Hepatics.  —  By    far    the    larger    number    of 

Hanson.  hepatics    belong   to    this    order.      They    are 

especially  abundant  in  moist  tropical  countries,  where  they  often 
cover  the  stems  and  leaves  with  a  rich  vegetation,  and  with  us 
they  are  of  common  occurrence  on  dripping  rocks  and  in  deep 
woods  on  the  moist  bark  of  trees  and  decaying  logs  or  damp 
earth.  In  the  lower  form  the  thallus  is  very  simple  and  delicate 


DEVELOPMENT   OF   PLANTS 


289 


(Fig.  196).  In  other  genera  there  are  indications  of  a  lobing  in 
the  thallus  which  becomes  more  pronounced  in  some  forms  and 
leads  by  gradual  gradations  to  genera  in  which  the  lobes  appear 
as  distinct  leafy  organs  arranged  in  two  rows  upon  a  stem-like 


FIG.  197.  One  of  the  leafy  Jungermaniales,  Porella:  A,  branch  of  the 
plant  bearing  several  sporophytes.  B,  under  surface  of  a  branch,  showing 
the  lobing  of  the  leaves  and  a  row  of  minute  scale-like  leaves.  C,  portion 
of  a  branch  bearing  an  archegonium  surrounded  by  cup-like  perianth  with 
minute  involucrate  leaves  at  base.  D,  branch  with  cone-like  antheridial 
branchlets.  At  the  left  a  single  leaf  is  shown  with  globular  antheridium 
in  its  axis.  E,  section  of  a  branch  similar  to  C.  The  archegonium  is  seen 
surrounded  by  the  perianth  and  below  the  involucrate  leaves.  The  sporo- 
phyte  is  nearly  mature  and  ready  to  elongate.  It  consists  of  a  round  cap- 
sule containing  elaters  and  spore  mother  cells  that  are  dividing  to  form  four 
spores  each.  Below  the  capsule  is  the  stalk  or  seta  which  ends  in  a  foot 
buried  in  the  tissues  of  the  branch.  At  the  right  is  an  unfertilized  arche- 
gonium that  shows  the  original  position  and  size  of  this  organ. — H.  O.  Hanson. 


290  STRUCTURE   OF   LEAFY    HEPATICS 

axis  (Fig.  197).  Usually  a  third  row  of  rudimentary  leaves, 
associated  with  rhizoids,  appears  upon  the  under  side  of  the  thal- 
lus  (Fig.  197,  B).  Although  the  Jungermaniales  tend  to  become 
highly  modified  into  a  leafy  body  the  tissues  remain  practically 
unchanged  in  all  forms.  Even  in  the  leafy  genera,  which  com- 
prise the  majority  of  forms  in  this  order,  there  is  little  evidence 
of  such  a  differentiation  of  the  tissues  as  characterize  the  Mar- 
chantiales.  Without  doubt,  the  simple  thallose  Jungermaniales 
are  the  most  primitive  forms  of  the  hepatics  and  probably  stand 
nearer  the  ancestral  type  than  any  other  form.  The  Marchan- 
tiales  represent  a  line  in  which  the  tissues  of  the  thallus  have 
attained  a  considerable  degree  of  differentiation,  but  in  the 
present  order,  the  evolution  led  to  a  marked  change  of  form  with 
but  slight  modification  of  the  tissues.  This  is  apparent  in  the 
highest  forms  where  the  leaves,  consisting  of  little  more  than  a 
single  layer  of  chlorophyll-bearing  cells,  are  arranged  in  two 
rather  oblique  rows  upon  delicate  stems.  The  under  surfaces 
of  these  leaves  are  generally  lobed  and  often  form  sacs  con- 
taining water  (Fig.  197,  B).  This  peculiarity  of  the  leaves, 
together  with  the  rudimentary  third  row  of  leaves  that  are 
associated  with  the  rhizoids,  makes  a  sharp  contrast  between 
the  upper  and  lower  surfaces  and  serves  to  distinguish  them 
from  the  mosses,  with  which  they  are  often  confused.  In  fact, 
they  are  often  called  scale  mosses,  owing  to  the  moss-like  ap- 
pearance of  their  dorsal  surface  and  the  close  contact  formed 
with  bark  or  other  surfaces  over  which  they  creep.  This  habit 
of  pressing  the  leaves  against  the  substratum  tends  to  retain 
the  moisture  and  also  enables  the  leaves  to  absorb  it  directly 
through  their  delicate  cell  walls.  The  overlapping  of  the 
leaves  in  many  genera,  as  well  as  their  lobing,  would  serve  the 
same  purpose  and  enable  the  plants  to  live  under  drier  conditions 
than  would  otherwise  be  possible.  Doubtless,  these  departures 
have  been  of  great  advantage  to  the  plants  and  in  part  account 
for  the  common  occurrence  of  these  leafy  forms.  It  should  be 
stated  that  the  leafy  hepatics  are  regarded  as  forms  that  have 
been  evolved  from  the  simple  thallose  forms  in  quite  recent 
geological  times  and  owing  to  their  better  adaptation  to  present 


DEVELOPMENT  OF  PLANTS  291 

conditions  they  have  become  the  most  numerous  of  all  the 
Hepaticae.  The  cause  of  the  differentiation  of  the  tissues  in 
the  two  groups  is  doubtless  explained  by  their  relation  to  mois- 
ture. The  leafy  hepatics  are  easily  wetted  and  the  entire  plant 
body  can  readily  absorb  moisture.  Therefore  the  tissues  remain 
delicate  and  fewer  rhizoids  are  required  to  absorb  the  moisture. 
The  Marchantiales  are  not  wettable,  and  as  a  consequence  they 
must  develop  wick-like  strands  of  rhizoids  as  well  as  the  elabo- 
rate storage  tissues  of  the  thallus.  Both  groups,  and  indeed  all 
hepatics,  must  remain  prostrate,  because  they  are  dependent 
upon  surface  water  and  have  not  as  yet  developed  an  absorbing 
and  conducting  apparatus  that  will  permit  of  any  other  position. 
Reproductive  Features  of  the  Jungermaniales . — Asexual  repro- 
duction is  of  the  same  vegetative  type  as  has  been  seen  in  the 
preceding  groups.  The  archegonia  and  antheridia  are  also  of 
essentially  the  same  character  as  seen  in  Marchantiales.  They 
are  borne  upon  the  dorsal  surface  of  the  thallus  or  upon  more 
or  less  modified  branches,  the  archegonia  often  arising  upon 
the  apex  of  the  branch  and  the  antheridia  appearing  as  rather 
spherical  bodies  in  the  axis  of  the  leaves  (Fig.  197,  C,  D).  The 
archegonia  are  developed  in  rather  conspicuous  cup-like  or  leafy 
outgrowths  of  the  thallus.  This  structure,  termed  the  perianth 
(Fig.  197,  C),  assumes  very  characteristic  forms  in  the  different 
genera  and  is  generally  associated  with  modified  leaves,  the 
involucre.  By  these  devices  the  archegonia  gain  the  same 
protection  against  drying  out  as  was  secured  to  them  by  the 
cavities  in  which  they  were  developed  in  the  preceding  group. 
The  gametospore  develops  after  the  manner  noted  in  Marchantia, 
forming  a  capsule  with  elaters,  but  its  stalk  or  seta  reaches  much 
larger  dimensions,  owing  doubtless  to  the  well-developed  foot 
(Fig.  197,  E).  The  majority  of  the  genera  also  are  characterized 
by  having  many  of  the  cells  of  the  capsule  sterilized  so  that 
the  number  of  spore-producing  cells  becomes  further  reduced. 
When  the  spores  have  been  matured  the  seta  rapidly  elongates 
to  several  times  its  original  length,  rupturing  the  archegonium 
and  lifting  the  capsule  high  in  the  air  (Figs.  196;  197,  A). 
The  capsule  usually  breaks  open  into  four  valves  which  are  hygro- 


292  DEVELOPMENT   OF  ANTHOCEROS 

scopic,  closing  over  the  spores  in  damp  weather  and  opening 
in  dry  weather  to  expose  them  to  the  wind.  These  spores  germi- 
nate and  begin  the  life  history  of  a  new  gametophyte.  As  in 
some  of  the  Marchantiales  a  filamentous  alga-like  growth  is 
first  formed  by  the  germinating  spore  before  the  characteristic 
plant  is  reproduced. 

1 06.  Order  c.  Anthocerotales  or  Horned  Liverworts. — This 
small  group  of  four  genera  is  the  most  interesting  of  any  of  the 
hepatics  because  it  presents  features  that  ase  suggestive  of  the 
algae  and  also  of  a  relationship  with  the  mosses.  The  thallus 
is  of  a  primitive  type,  often  with  simple  lobings  and  therefore, 
suggestive  of  relationship  with  the  simpler  Jungermaniales  (Fig. 
198).  A  peculiar  feature  of  the  order  is  the  occurrence' of  mucil- 


FIG.  198.  One  of  the  Anthocerotales,  Anthoceros,  bearing  four  pod-shaped 
sporophytes,  s.  The  one  on  the  right,  s,  has  opened  at  the  top  and  is  dis- 
charging the  spores,  but  elongation  and  the  formation  of  spores  continue 
below  owing  to  its  basal  growth. 

age  cavities  which  communicate  with  the  air  through  small 
openings  on  the  under  side  of  the  thallus.  These  cavities  are 
always  occupied  by  one  of  the  blue-green  algae,  Nostoc,  which 
possibly  assists  the  plant  in  the  retention  of  water  owing  to  their 
mucilaginous  character.  Another  interesting  feature  of  the  order 
is  the  occurrence  usually  of  a  single  chloroplast  in  each  cell,  as  in 
Coleochaete  and  several  other  genera  of  the  algae.  The  archegonia 
and  antheridia,  while  resembling  those  of  the  preceding  orders, 


DEVELOPMENT  OF  PLANTS  293 

are  sunken  in  the  tissues.  This  arrangement  doubtless  protected 
the  reproductive  organs  just  as  did  the  involucre  and  perianth 
in  the  preceding  groups. 

The  most  suggestive  variation  of  the  Anthocerotales  appears 
in  the  development  of  the  sporophyte.  We  have  noted  that  the 
asexual  generation  in  forms  like  Ricciocarpus  was  a  microscopic 
plant  consisting  of  a  delicate  spore-bearing  capsule.  In  Mar- 
chantia  the  sporophyte  is  still  minute  but  differentiated  into  a 
foot,  stalk  and  capsule,  and  in  the  Jungermaniales  this  becomes 
much  more  pronounced.  In  the  Anthocerotales  the  sporophyte 
assumes  a  growth  and  differentiation  that  is  much  more  extensive 
and  complex  and  it  also  presents  features  that  indicate  a  rela- 
tionship with  the  mosses.  Furthermore  it  shows  certain  features 
in  its  development  that  also  arose  in  the  ferns.  The  germination 
of  the  gametospore  gives  rise  to  a  rather  cylindrical  or  pod-shaped 
body  (Fig.  199),  in  which  all  the  cells  have  become  sterile  save  a 
dome-shaped  layer.  The  basal  portion  of  the  sporophyte  de- 
velops into  a  massive  foot,  often  provided  with  rhizoidal-like 
outgrowths,  which  serve  as  a  very  efficient  absorbing  organ. 
The  upper  portion  of  the  sporophyte  presents  a  remarkable  series 
of  differentiations.  The  outer  part  of  it  consists  of  chlorophyll- 
bearing  cells  in  which,  for  the  first  time,  genuine  stomata  appear 
(Fig.  199,  ch).  Within  this  zone  of  chlorenchyma  is  a  dome- 
shaped  layer  of  spore  mother  cells  often  alternating  with  sterile 
cells  which  in  some  genera  develop  as  elaters.  In  the  center  of 
the  sporophyte  is  a  mass  of  rather  elongated  cells,  the  columella, 
which  assist  in  conducting  food  from  the  foot  to  the  forming 
spores.  Directly  above  the  foot  is  a  region  of  rapidly  dividing 
cells  which  causes  the  sporophyte  to  elongate  by  basal  growth  and 
push  out  of  the  archegonium  before  the  spores  are  mature.  This 
is  a  radical  departure  from  the  other  liverworts  in  the  mode  of 
development  of  the  sporophyte  and  we  will  see  essentially  the 
same  manner  of  growth  occurring  in  the  mosses.  As  the  sporo- 
phyte develops,  the  spores  in  the  upper  part  of  it  mature  and  the 
sporophyte  splits  into  two  valves  (Fig.  198,  s'),  thus  permitting 
the  scattering  of  the  spores.  The  lower  portion,  however,  may 
continue  to  elongate  for  several  months  in  some  species,  forming 
additional  spores. 


294 


SPOROPHYTE  OF  ANTHOCEROS 


It  is  evident  that  this  sporophyte  would  become  a  self-support- 
ing plant  if  its  foot  should  reach  through  the  gametophyte  and 
absorb  substances  from  the  soil.  In  comparing  the  sporophyte 


FIG.  199.  FIG.  200. 

FIG.  199.  Section  of  a  young  sporophyte  of  Anthoceros  emerging  from 
the  involucre-like  outgrowth  of  the  thallus — sp,  dome-shaped  spore  form- 
ing layer  of  cells;  ch,  chlorenchyma  with  stomata;  b,  foot  or  absorbing  region; 
c,  region  of  growth.  -At  right,  surface  view  of  stoma. 

FIG.  200.  A  common  moss,  Funaria:  A,  two  plants  with  root-like  rhi- 
zoids  at  base  and  radially  arranged  leaves.  Rising  above  the  leaves  are 
the  stalks  or  setae  and  capsules  of  two  sporophytes.  B,  magnified  view  of 
a  plant,  showing  the  early  appearance  of  the  sporophytes  as  a  delicate  stalk 
still  covered  by  the  enlarging  archegonium  or  calyptra.  C,  a  plant  bearing 
antheridia  in  a  rosette  of  leaves  at  apex  of  stem.  D,  enlarged  view  of  the 
upper  portion  of  the  sporophyte,  showing  the  twisting  of  the  stalk  that  assists 
in  sifting  the  spores  through  the  fringe  of  teeth,  peristome,  that  encircles  the 
mouth  of  the  capsule. 

in  forms  like  Ricciocarpus,  Marchantia  and  the  leafy  hepatics 
with  Anthoceros,  we  see  that  it  has  undergone  a  gradual  evo- 
lution and  has  finally  reached  a  point  where  it  only  needs  to 


DEVELOPMENT   OF   PLANTS  295 

come  in  contact  with  the  soil  to  become  an  independent  plant 
since  it  is  provided  with  all  the  tissues  necessary  for  photosyn- 
thesis and  absorption.  Note  also  that  the  growth  of  the  sporo- 
phyte  is  becoming  more  prolonged,  that  the  spore  mother  cells 
appear  later  in  its  development  and  comprise  but  a  small  portion 
of  it,  and  finally,  that  the  sporogenous  and  sterile  tissues  alter- 
nate, forming  a  dome-like  zone  of  cells.  Structures  very  sugges- 
tive of  these  features  will  appear  among  the  mosses  and  ferns. 

Class  B.    Musci  or  Mosses 

107.  General  Characteristics. — The  mosses  are  by  far  the 
largest  group  of  the  Bryophyta  and  show  marked  advances  over 
the  hepatics  in  the  development  of  the  gametophyte  and  sporo- 
phyte.  Particularly  noticeable  is  the  size  and  differentiation  of 
the  sporophyte  which  now  becomes  among  the  majority  of  the 
genera  of  nearly  equal  importance  with  the  gametophyte  (Fig. 
200) .  Variations  appear  in  the  mosses  that  have  been  successful 
in  adapting  them  to  a  great  variety  of  conditions,  ranging  from 
submerged  aquatics  to  pronounced  xerophytes  that  live  upon 
exposed  rocks.  As  a  result,  they  are  of  common  occurrence 
everywhere.  The  mosses,  however,  do  not  show  any  variations 
that  have  led  to  higher  types  of  plants.  In  fact,  they  are  re- 
garded as  a  highly  specialized  group,  like  the  Jungermaniales, 
that  have  branched  off  from  the  hepatics  in  recent  geological 
times  and  become  the  dominant  representa  tives  of  the  Bryophyta 
owing  to  their  better  adaptation  to  present  conditions  upon  the 
earth.  This  recent  derivation  of  the  mosses  is  indicated  by  the 
remarkable  uniformity  of  structure  that  characterizes  the  entire 
group. 

While  the  plants  themselves  are  relatively  small,  they  often 
form  large  swards  or  mats,  owing  to  the  extensive  branching 
and  prolonged  growth  of  the  stems  and  their  rapid  multiplication 
through  the  dying  off  of  the  older  parts  and  the  independent 
growth  of  the  branches  that  are  thus  set  free.  Owing  to  this 
habit  of  growth  and  multiplication  the  mosses  often  furnish  the 
conspicuous  features  of  the  vegetation.  Especially  is  this  true 
in  northern  regions  and  in  bogs  and  barrens  where  great  sections 


296  NATURE   OF   BOG   MOSSES 

of  the  country  are  carpeted  with  them.  The  mosses  are  leafy 
stemmed  plants  like  the  Jungermaniales,  but  they  usually 
possess  the  decided  advantage  of  having  erect  stems  and  radi- 
ally arranged  leaves  (Fig.  200).  Even  in  the  prostrate  forms 
where  but  two  rows  of  leaves  can  be  developed,  the  resemblance 
to  the  leafy  hepatics  is  only  superficial.  The  moss  leaves  are 
not  lobed  and  the  central  portion  is  usually  traversed  by  a  strand 
of  cells  which  serve  for  conduction  like  the  vascular  bundles  of 
higher  plants  (Fig.  206,  B}.  The  differentiation  of  the  tissues 
of  the  stem  also  shows  a  marked  advance  over  preceding  forms. 
A  central  conducting  region  of  elongated  cells  that  may  be 
compared  to  a  rudimentary  vascular  system  and  a  cortical  zone, 
often  with  thickened  cells  and  rudimentary  epidermis,  are  fre- 
quently to  be  seen  (Fig.  206,  A).  Asexual  reproduction  is  al- 
most entirely  confined  to  the  detachment  of  branches  as  stated 
above.  In  only  a  few  genera  have  the  formation  of  gemmae  been 
noticed.  The  sexual  reproduction  and  the  development  of  the 
sporophyte  present  some  interesting  features  that  will  be  noticed 
in  the  following  orders. 

1 08.  Order  a.  Sphagnales.  Bog  or  Peat  Mosses. — A  single 
genus,  Sphagnum,  of  numerous  poorly  defined  species  is  the  sole 
representative  of  this  order  and  in  several  respects  it  occupies 
an  intermediate  position  between  the  hepatics  and  the  mosses 
proper.  These  pale-green  mosses  (Fig.  201)  grow  on  bogs  and 
moors  and  other  places  where  they  are  subject  to  drainage  con- 
taining organic  matter,  as  humic  acid,  derived  from  the  decay  of 
plant  and  animal  life.  The  majority  of  plants  are  unable  to 
endure  these  conditions,  which  are  popularly  referred  to  as  sour, 
and  as  a  result,  you  will  always  find  associated  with  the  bog 
mosses  a  rather  limited  and  peculiar  variety  of  plants  such  as 
several  genera  of  heaths,  sedges,  orchids,  pitcher  plants  (Sar- 
racenia)  and  other  insectivorous  plants  like  the  sundew  (Drosera) . 
For  some  reason  the  ordinary  plant  is  not  able  to  procure  its  food 
from  these  sour  bogs  and  this  may  explain  the  common  occurrence 
in  such  places  of  insectivorous  plants,  and  of  certain  trees  and 
shrubs  that  are  associated  with  mycorrhiza.  The  sphagnums 
grow  luxuriantly  in  such  places,  the  lower  portions  of  the  stem 


DEVELOPMENT   OF   PLANTS 


297 


dying  off  and  the  upper  portion  branching  and  continuing  the 
growth  from  year  to  year.  In  this  way,  ponds  are  gradually 
covered  with  a  layer  of  mosses  which  become  rather  insecurely 
bound  together  by  the  subsequent  introduction  of  other  plants 
whose  roots  or  stems  traverse  the  covering  formed  by  the  mosses. 
So  frail  is  this  covering  at  first,  that  a  very  slight  jar  will  cause 
the  surface  to  tremble  and  for  this  reason  these  places  are  known 


FIG.  201. 


FIG.  202. 


FIG.  201.  The  bog  moss,  Sphagnum,  bearing  three  sporophytes  and 
numerous  lateral  branches  covered  with  closely  overlapping  leaves.  Note 
that  some  of  these  branches  envelop  the  stem  in  wick-like  strands. 

FIG.  202.  Structure  of  the  leaf  of  Sphagnum:  A,  section  of  leaf,  showing 
its  single  layer  of  cells  that  consists  of  large  empty  cells  alternating  with 
small  chlorophyll-bearing  ones.  Bt  surface  view  of  the  cells,  the  larger  empty 
cells  being  marked  with  spiral  bands  and  often  perforated  with  minute  open- 
ing. 

as  quaking  bogs.  The  constant  dropping  of  the  decaying  vege- 
tation to  the  bottom  of  the  ponds,  assisted  by  the  drainage 
material,  gradually  fills  them,  and  in  time  they  may  become 
quite  dry.  The  acid  character  of  these  bogs  prevents  the  entrance 
of  those  organisms  which  promote  decay.  As  a  result,  plants 
20 


298  STRUCTURE   OF  SPHAGNUM 

and  animals  that  fall  into  these  bogs  may  only  partly  decay  and 
remain  for  ages  in  a  wonderful  state  of  preservation  as  is  shown 
by  the  recovery  of  skeletons  of  animals  and  clothing  of  men  that 
belonged  to  a  prehistoric  period.  It  is  because  of  this  condition 
that  the  bog  mosses  are  of  great  economic  importance.  The 
dead  portions  of  the  moss  plants,  as  well  as  that  of  the  associated 
plants,  do  not  entirely  decay,  and  consequently  there  is  slowly 
formed  a  compact  mass  of  material  rich  in  carbon.  This  is  cut 
or  pressed  into  blocks,  forming  peat.  This  material  is  an  im- 
portant fuel  in  Europe  and  the  vast  deposits  of  it  in  this  country 
will  doubtless  be  utilized  in  the  future.  It  has  been  recently 
shown  that  the  material  of  some  of  these  bogs  is  of  great  value 
as  a  source  of  pulp  for  paper. 

(a)  The  Structure  and  Reproduction  of  Sphagnum. — The  leaves 
of  the  bog  moss  are  arranged  in  compact  spirals  around  the 
stems  and*  consist  of  a  single  layer  of  cells  as  in  the  leafy  hepatics 
(Fig.  202,  E).  These  cells,  however,  are  of  two  kinds,  large 
and  empty  cells  with  spirally  thickened  walls  which  are  generally 
perforated  with  small  pores  and  very  narrow  cells  containing 
chlorophyll.  This  distribution  of  the  cells  explains  the  pale~ 
green  color  characteristic  of  the  sphagnums.  Large  and  spirally 
marked  cells,  similar  to  those  of  the  leaf,  may  also  occur  in  the 
cortex  of  the  stem.  The  closely  packed  leaves  enable  the  bog 
mosses  to  take  up  water  like  a  sponge  and  the  sphagnums  are 
of  considerable  commercial  value  for  this  reason,  being  exten- 
sively employed  by  horticulturists  to  keep  plants  moist  during 
shipment.  They  are  also  used  extensively  in  stables  in  place 
of  straw  and  they  render  them  almost  odorless  owing  to  their 
absorption  of  liquids  and  gases.  Perhaps  these  large  cells  may 
serve  as  floats,  enabling  the  plant  to  bridge  over  ponds  and  they 
may  enable  the  plant  to  endure  the  acid  waters  in  which  these 
plants  grow.  In  this  connection,  it  is  noteworthy  that  rhizoids 
are  entirely  lacking  and  that  water  and  other  substances  are 
absorbed  by  the  outer  cells  of  the  stems  and  by  certain  branches 
which  hang  down  in  wick-like  strands  close  to  the  main  stem 
(Fig.  201). 

The  antheridia  and  archegonia  are  developed  much  as  in  the 


DEVELOPMENT   OF   PLANTS 


299 


leafy  hepatics,  the  former  organs  appearing  as  stalked  bodies 
in  the  axils  of  the  leaves  on  short  cone-like  branches  and  the 
archegonia  originate  on  the  tips  of  short  branches,  The  gameto- 
spore  germinates  very  much  as  in  Anthoceros  (Fig.  203,  6). 
It  does  not,  however,  have  as  prolonged  a  growth,  and  at  ma- 


FIG.  203.  The  sporophyte  of  Sphagnum:  6,  the  young  sporophyte  sepa- 
rated from  the  archegonium  with  essentially  the  same  differentiation  of 
parts  as  noted  in  Anthoceros,  Fig.  199.  5^4,  diagram  of  a  later  development 
of  the  sporophyte  in  the  archegonium.  The  enlarged  foot,  b,  is  embedded 
in  the  apex  of  the  moss  branch,  p,  and  the  spores,  sp,  form  a  dome-shaped 
layer  in  the  upper  part  of  the  capsule.  5,  the  naked  stem,  p,  of  the  moss 
branch,  surrounded  at  base  with  spirally  arranged  leaves,  has  elongated, 
lifting  the  mature  sporophyte  into  the  air.  The  enlargement  of  the  stem, 
b,  is  due  to  the  growth  of  the  foot  region  of  the  sporophyte;  ca,  remains  of 
the  ruptured  archegonium  or  calyptra;  o,  lid  or  operculum  of  the  capsule. 
— After  Schimper. 

turity  consists  of  a  well-developed  foot  embedded  in  the  tissues 
of  the  gametophyte  and  a  spore-bearing  capsule  (Fig.  203,  5^4). 
The  stomata  and  chlorenchyma  which  were  so  conspicuous  in 
Anthoceros  are  less  perfectly  represented,  but  the  spore  mother 
cells  are  still  developed  in  a  dome-shaped  zone.  This  rather 
minute  sporophyte  at  maturity  barely  breaks  through  the  arche- 
gonium, known  in  this  condition  as  the  calyptra,  which  covers 
the  capsule  as  a  cap.  The  absence  of  a  conspicuous  stalk  or  seta 


300 


SPOROPHYTE   OF   SPHAGNUM 


in  the  sporophyte  of  Sphagnum  is  provided  for  by  the  elongation 
of  the  upper  part  of  the  moss  stem  which  pushes  the  sporophyte 
above  the  leaves  and  exposes  the  capsule  to  the  winds  quite  as 


FIG.  204.  FIG.  205. 

FIG.  204.  Germination  of  the  spore  of  Sphagnum:  A,  early  growth  of 
the  spore.  B,  later  development — sp,  spore;  b,  bud  developing  into  moss 
plant.  C,  margin  of  the  thallus,  showing  the  origin  of  the  bud. — After  Camp- 
bell. 

FIG.  205.  The  twisted  stalk  moss,  Funaria:  A,  two  sporophyte-bearing 
plants,  the  remains  of  the  archegonium  or  calyptra  still  attached  to  the  cap- 
sule on  the  right.  B,  enlarged  view  of  plant  with  the  young  sporophyte, 
still  enclosed  in  the  archegonium,  just  emerging  above  the  leaves.  C,  the 
male  plant  bearing  the  antheridia  in  a  conspicuous  rosette  of  leaves.  D, 
upper  portion  of  the  sporophyte,  showing  the  twisting  of  the  stalk  or  seta 
that  assist  in  sifting  the  spores  through  the  teeth,  peristome,  that  encircle 
the  mouth  of  the  capsule. 

effectually  as  the  seta  of  the  hepatics  and  mosses  (Fig.  203,  5). 
The  spores  are  freed  by  the  forcing  off  of  a  circular  lid,  the 
operculum,  as  is  the  case  among  the  majority  of  the  mosses 
(Fig.  203,  0).  This  is  effected  by  a  ring  of  thin-walled  cells, 


DEVELOPMENT   OF   PLANTS  301 

the  annulus,  that  lies  in  the  groove  just  below  the  operculum, 
As  the  walls  of  the  capsule  dry  out  at  maturity  they  shrink, 
compressing  the  air  within  the  capsule  until  the  tension  ruptures 
the  delicate  ring  of  cells  and  throws  out  the  spores  with  an 
audible  report.  No  elaters  are  associated  with  the  spores  in  any 
of  the  mosses. 

The  germination  of  the  spores  results  in  the  formation  of  a 
short  chain  or  filament  of  cells  which  soon  develop  into  a  flat 
and  rather  irregular  thallus  consisting  of  a  single  layer  of  cells 
and  attached  to  the  ground  by  numerous  rhizoids  (Fig.  204). 
This  thallus  may  produce  from  its  marginal  cells  other  filaments 
which  behave  like  the  original  one  and  thus  serve  to  increase  the 
number  of  thalli.  The  archegonia  and  antheridia  are  not  de- 
veloped upon  these  thalli,  but  there  are  first  formed  as  in  most 
of  the  Jungermaniales,  bud-like  growths  which  develop  into  leafy 
stems  of  the  sphagnum  (Fig.  204,  &).  These  stems  soon  become 
independent  plants  through  the  decay  of  the  thallus  and  finally 
bear  the  reproductive  organs. 

109.  Order  b.  Bryales.  The  Higher  or  True  Mosses. — The 
great  majority  of  plants,  commonly  known  as  mosses,  belong 
to  this  order.  As  a  rule,  these  plants  are  of  a  higher  type  than 
any  of  the  other  Bryophyta  (Fig.  205) .  The  stems  are  frequently 
erect  with  radially  arranged  leaves  and  show  considerable  differ- 
entiation of  tissues,  as  is  apparent  in  the  cross-section  of  the  stem 
of  one  of  the  higher  forms,  shown  in  Fig.  206,  A.  Particularly 
noticeable  are  the  thin-walled  cells  located  in  the  central  strand 
of  the  stem  which  serve  to  conduct  the  fluids  and  are  therefore, 
comparable  to  the  tracheal  tissue  of  the  higher  plants.  A  similar 
conducting  tissue  is  often  seen  in  the  central  strand  that  appears 
in  the  leaves  of  the  majority  of  the  Bryales  (Fig.  206,  B).  In 
connection  with  this  higher  development  of  the  plant,  attention 
should  be  directed  to  the  more  efficient  organs  of  absorption. 
The  rhizoids  that  spring  from  the  base  of  the  erect  stems  or  from 
the  under  side  of  forms  with  creeping  stems  are  often  larger  than 
those  noted  in  the  hepatics,  and  generally  they  are  multicellular 
and  often  twisted  together  into  root-like  strands  (Fig.  206,  C). 
These  features  doubtless  account  in  part  for  the  larger  size  that 


302 


STRUCTURE   OF  THE   BRYALES 


is  attained  by  many  of  the  Bryales.  It  is  noteworthy,  however, 
that  the  devices  for  absorption  and  conduction  of  water  are  not 
as  yet  sufficiently  developed  to  enable  these  plants  to  attain 
any  considerable  height.  Erect  forms  rarely  exceed  a  few  centi- 
meters in  height,  but  prostrate  forms  creep  over  the  ground  in- 
definitely. Some  mosses  are  short  lived  and  many  are  per- 
ennial, continuing  their  apical  growth  from  year  to  year.  Some 
can  live  only  in  the  wettest  of  places  or  as  submerged  aquatics, 


\ 


FIG.  206.  Structural  features  of  the  moss  plant;  A,  cross-section  through 
the  basal  region  of  a  moss  stem,  showing  rhizoidal  outgrowths  from  epi- 
dermis, cortical  and  central  conducting  tissues.  B,  leaf  of  Mnium  with  mid- 
vein  of  elongated  conducting  cells.  C,  rhizoids  twisted  together  into  root- 
like  strands.  D,  cross-section  of  a  leaf  of  Polytrichum,  showing  the  partial 
folding  in  of  the  margins  of  the  leaf  to  protect  the  delicate  plates  of  chloro- 
phyll-bearing cells  against  drought. 

while  others  are  capable  of  enduring  almost  complete  desiccation 
and  revive  quickly  when  moistened.  An  interesting  device  of 
service  in  adapting  these  plants  to  periods  of  drought  appears 
in  the  leaves  of  some  of  the  higher  forms,  as  the  hair  cap  moss, 
Polytrichum.  Plates  of  green  cells  parallel  to  the  midvein  project 
from  the  upper  surface  of  the  leaves,  thus  greatly  increasing  the 
surface  of  the  leaves  for  photosynthesis  and  also  serving  as  water 


DEVELOPMENT   OF   PLANTS 


303 


7     X^^X  6 

FIG.  207.  Male  reproductive  organs:  I,  male  plants  of  Polytrichum 
bearing  antheridia  in  cup-like  buds.  6,  section  of  apex  of  stem  of  Funaria 
— an,  antheridia;  p,  paraphysis.  7,  male  gametes,  the  one  at  left  still  en- 
veloped in  mucilaginous  remains  of  mother  cell  wall. 


FIG.  208.  Female  reproductive  organs:  8,  longitudinal  section  of  apex 
of  stem,  showing  arrangement  of  archegonia.  9,  archegonium  enlarged, 
the  canal  cells  just  beginning  to  dissolve — g,  the  female  gamete. 


304 


SPOROPHYTE   OF  THE   BRYALES 


reservoirs  (Fig.  206,  D).  During  drought,  this  delicate  apparatus 
is  protected  by  the  coiling  of  the  leaves  into  needle-like  rolls 
which  result  in  the  exposure  only  of  the  thickened  epidermal  cells. 
The  sexual  organs  are  developed  on  the  apices  of  the  main  or 
lateral  branches  and  they  are  protected  by  modified  leaves  which 
are  often  colored  and  form  a  more  or  less  conspicuous  bud  or 
cup  (Fig.  207,  i).  This  is  especially  true  of  the  antheridial 
buds,  the  plants  being  monoecious  or  dioecious.  The  antheridia 
and  archegonia  are  essentially  of  the  same  structure  as  noted  in 


FIG.  209.  Germination  of  the  gametospore :  10,  base  of  the  archegonium 
in  which  the  gametospore  has  germinated,  forming  a  mass  of  cells  with  apical 
growing  cell,  x.  n,  later  growth  of  the  gametospore.  The  sporophyte, 
spr,  still  enveloped  by  the  archegonium,  ar,  appears  as  a  cylindrical  mass 
of  cells  with  foot,  b,  penetrating  the  stem  of  the  moss  plant.  At  right  an 
unfertilized  archegonium.  See  Fig.  205,  B. — After  Sachs. 

previous  groups  and  they  are  usually  associated  with  modified 
leaves  known  as  paraphyses  (Figs.  207,  6;  208).  The  germina- 
tion of  the  gametospore  and  the  development  of  the  sporophyte, 
while  presenting  many  features  in  common  with  the  hepatics 
and  Anthoceros  in  particular,  shows  a  remarkable  series  of  varia- 
tions that  are  of  decided  advantage  to  the  plant.  The  gameto- 
spore in  its  early  growth  forms  a  spindle-shaped  mass  of  cells, 


DEVELOPMENT   OF   PLANTS  305 

the  basal  portion  of  which  reaches  down  into  the  stem  of  the 
moss  plant  and  forms  a  well-developed  absorbing  organ  or  foot, 
while  the  upper  portion  elongates  by  means  of  an  apical  cell 
(Fig.  209,  10).  Later,  the  growth  becomes  basal  as  in  Antho- 
ceros.  For  a  time,  the  archegonium  keeps  pace  with  the  elonga- 
tion of  the  young  sporophyte  (Fig.  209,  n),  but  finally  it  is 
ruptured  and  lifted  up  as  a  cap,  called  the  calyptra,  on  the  apex 
of  the  young  sporophyte.  This  is  one  of  the  most  essential 
differences  between  the  mosses  and  hepatics.  In  the  latter  group, 
with  the  exception  of  the  An thocero tales,  the  spores  were  matured 
in  the  archegonium.  In  the  Bryales,  the  spores  are  usually 
not  formed  until  the  seta  has  elongated  considerably.  As  the 
sporophyte  elongates,  the  upper  part  enlarges  and  finally  forms 
a  complex  capsule  which  remains  covered  for  a  varying  length 
of  time  by  the  calyptra.  The  hairy  calyptra  of  Polytrichum 
(this  name  means  many  hairs)  is  due  to  the  development  of  a 
felt  of  hairs  upon  the  archegonium  which  forms  a  protective 
covering  to  the  young  sporophyte  against  the  loss  of  water. 

The  capsule  and  calyptra  assume  various  forms  and  positions. 
In  some  cases  the  capsule  is  quite  erect  and  completely  covered 
by  the  calyptra  (Fig.  210,  i),  and  again  it  may  be  more  or 
less  inclined  or  even  pendulous  and  the  calyptra  laterally  placed 
like  a  cap  (Figs.  205,  A;  211,  5).  Removing  the  calyptra  you 
notice  that  a  lid  or  operculum  is  situated  on  top  of  the  capsule 
while  at  the  bottom  is  a  somewhat  enlarged  region,  the  apophysis, 
provided  with  stomata  that  communicate  with  the  interior  re- 
gions of  the  capsule  (Figs.  210,  2;  211,  5).  When  the  opercu- 
lum is  removed  a  circle  of  minute  teeth-like  structures,  the  peri- 
stome,  appears  that  more  or  less  completely  closes  the  mouth  of 
the  capsule  (Fig.  205,  D).  In  some  mosses  a  delicate  membrane, 
the  epiphragm,  lies  just  below  the  peristome,  which  assists  in 
closing  the  mouth  of  the  capsule  (Fig.  210,  2B>  4^4).  We  are 
now  interested  to  know  the  meaning  of  these  structures  and  to 
learn  how  they  have  come  about.  By  examining  a  longitudinal 
section  of  the  nearly  mature  eapsule  you  will  see  that  the  spore- 
producing  tissue  is  relatively  small  and  appears  as  a  hollow 
cylinder.  The  progressive  sterilization  that  we  have  seen  going 


306 


CAPSULE   OF   BRYALES 


on  has  here  resulted  in  the  transformation  of  the  upper  part  of 
the  dome-shaped  zone  of  sporogenous  cells  into  vegetative  cells 
(Figs.  210,  3;  211,  6).  The  sporogenous  tissue  is  usually  sur- 


FIG.  210.  The  mature  sporophyte  of  Polytrichum:  I,  moss  plant  bearing 
sporophyte.  iA,  calyptra  removed,  showing  capsule,  which  has  curved  to 
one  side.  2,  capsule,  the  calyptra,  2 A,  removed,  showing  the  operculum  or 
lid  and  the  enlarged  apophysis,  a,  at  base.  2J5,  capsule  with  operculum  re- 
moved, showing  the  teeth-like  peristome  and  the  epiphragm,  which  has 
been  lifted  up  at  one  side.  3,  section  of  nearly  mature  capsule — sp,  cyl- 
inder of  spore-forming  cells  that  is  surrounded  on  its  inner  and  outer  sides 
by  loosely  arranged  chlorophyll  cells;  o,  operculum;  r,  annulus;  p,  peri- 
stome; e,  epiphragm;  a,  apophysis.  3^4,  stoma  of  apophysis.  $B,  cross- 
section  of  capsule — sp,  spore-forming  cells.  ^A,  portion  of  mouth  of  cap- 
sule, showing  position  of  peristome  and  epiphragm  in  dry  weather.  4.8, 
peristome  pressed  down  on  epiphragm,  closing  the  capsule  in  damp  weather. 
— H.  O.  Hanson. 


DEVELOPMENT   OF   PLANTS 


307 


rounded  on  the  outside,  and  less  commonly  on  the  inside,  by 
loose  chlorophyll-bearing  cells  which  extend  down  to  the  base 
of  the  capsule  where  chlorenchyma  and  stomata  are  developed. 
This  lower  region  of  the  capsule,  the  apophysis,  is  often  enlarged 
and  provided  with  air  spaces  and  stomata,  thus  constituting  the 
chief  photosynthetic  apparatus  of  the  sporophyte  (Fig.  210, 


FIG.  211.  Structure  of  capsule  of  Funaria:  5,  capsule  with  calyptra,  §A, 
removed.  6,  section  of  nearly  mature  capsule — sp,  spore-forming  cells  sur- 
rounded on  outside  by  loosely  arranged  chlorophyll-bearing  cells;  o,  oper- 
culum;  r,  annulus;  p,  peristome;  a,  apophysis.  8,  magnified  view  of  a 
portion  of  the  capsule,  showing  the  annulus,  r,  and  the  thick-walled  cells 
of  the  peristome,  p,  which  are  attached  at  their  base  to  the  epidermis  by 
a  double  row  of  cells;  sp,  spore-forming  cells,  the  dotted  line  should  run  to  the 
four  roundish  cells  at  the  left.  7,  the  cells  shown  in  8,  p,  have  split  apart, 
thus  forming  the  inner  and  outer  teeth-like  segments  of  the  peristome. — After 
Sachs. 


308 


THE   PERISTOME   OF  THE   CAPSULE 


The  entire  surface  of  the  capsule  is  protected  by  a  well-developed 
epidermis  but  at  the  top  a  lid  or  operculum  is  cut  off  by  a  ring 
of  rather  delicate  mucilage-bearing  cells,  the  annulus,  which 
swells  at  maturity  and  so  assists  in  casting  off  the  operculum 
(Fig.  210,  3,  r).  Below  the  operculum  a  layer  of  cells,  known 
as  the  peristome,  extends  across  the  capsule  and  becomes  greatly 
thickened  on  its  outer  and  usually  on  its  inner  walls.  This  layer 
of  cells  is  firmly  attached  to  the  walls  of  the  capsule  by  rather 
short  thick-walled  cells  (Figs.  210,  3,  p;  211,  6-8,  p).  When 
the  spores  are  mature,  the  more  delicate  cells  break  down,  leaving 
little  more  than  a  loose  mass  of  spores  and  the  peristome  within 
the  capsule.  The  cells  of  the  peristome  break  apart  into  teeth - 


FIG.  212.  Germination  of  the  spore:  3,  early  stage  in  the  germination, 
4,  character  of  the  branching,  alga-like  filaments  that  are  finally  developed 
from  the  spore — r,  rhizoids  which  penetrate  the  ground;  b,  bud  which  will 
develop  into  a  leafy  moss  plant. 

like  segments  (Figs.  211,  7)  and  being  very  hygroscopic,  co- 
pperate  with  the  contracting  capsule  in  forcing  off  the  operculum. 
After  the  removal  of  the  operculum  the  mouth  of  the  capsule  is 
covered  by  the  peristome  which  now  appears  as  a  fringe  of  teeth 
radiating  out  from  the  walls  of  the  capsule  (Fig.  210,  4).  The 
teeth  of  the  peristome  are  always  in  multiples  of  two  and  form 
one  or  more,  usually  two  layers,  accordingly  as  only  the  outer  or 


DEVELOPMENT   OF   PLANTS  309 

both  surfaces  of  the  peristome  cells  are  thickened.  These  teeth 
are  so  variously  marked  and  fashioned  that  they  furnish  one  of 
the  most  characteristic  features  of  the  various  genera  of  the 
mosses.  The  peristome  closes  the  mouth  of  the  capsule  in  wet 
weather,  but  on  dry  days  it  opens  in  a  variety  of  ingenious  ways 
in  the  various  species  of  mosses  so  as  to  effect  a  gradual  scattering 
of  the  spores.  The  seta  also  contributes  in  the  distribution, 
being  very  elastic  and  often  hygroscopic  (Fig.  205,  D). 

The  spores  germinate  and  produce  profusely  branching  chains 
or  filaments  of  green  cells,  called  the  protonema  (Fig.  212).  In 
a  few  instances,  thalloid  structures  are  developed  as  in  Sphagnum. 
The  protonema  absorbs  the  earth  substances  through  delicate 
rhizoids  and  spreads  over  the  ground,  appearing  to  the  eye  as  a 
green  mould  or  fine  algal  cells.  Goebel  presents  some  evidence 
tending  to  show  that  the  mosses  may  have  been  derived  from 
branching  forms  of  the  green  algae.  Buds  finally  appear  on  these 
branches  at  numerous  places  and  develop  the  leafy  stems  of 
the  moss  plant  (Fig.  212,  b).  This  in  part  accounts  for  the  dense 
colonies  so  characteristic  of  the  mosses.  As  soon  as  the  young 
plants  are  established  in  the  soil  by  means  of  rhizoids  that  spring 
from  the  basal  parts,  the  protonema  withers  away,  although  in 
some  genera  it  is  perennial  and  continues  to  produce  new  plants 
from  year  to  year. 

no.  The  Relationship  of  the  Bryophyta. — It  appears  probable 
that  there  have  been  three  lines  of  variation  or  evolution  among 
the  Bryophyta  that  have  had  their  origin  in  some  simple  thalloid 
form  most  nearly  related  to  the  lower  Jungermaniales.  One  line 
has  resulted  in  the  highly  differentiated  thallus  of  the  Marchanti- 
ales  with  its  simple  sporophyte.  Another,  the  Jungermaniales, 
retaining  the  simple  structures  of  the  primitive  thallus  has  modi- 
fied its  form  into  a  leafy  plant  and  developed  a  more  complex 
sporophyte,  while  the  variations  of  the  third  line,  or  Anthocero- 
tales,  have  resulted  in  slight  alterations  of  the  primitive  type  of 
the  thallus  but  in  profound  modifications  of  the  sporophyte.  The 
variations  of  the  sporophyte  in  the  latter  group  appear  to  have 
been  very  advantageous  to  the  present  conditions  upon  the  earth 
and  perhaps  led  to  the  mosses.  The  important  feature  in  this 


310  EVOLUTION   OF  THE   SPOROPHYTE 

evolution,  already  noted  among  the  algae,  is  the  increasing  im- 
portance ot  the  sporophyte.  Among  the  green  algae  it  was  de- 
pendent upon  the  amount  of  food  stored  in  the  gametospore. 
In  the  Bryophyta,  it  is  better  nourished,  owing  to  its  essential 
parasitic  habit.  This  led  to  a  larger  growth  and  more  complex 
development  and  it  is  especially  important  to  note  that  the  sporo- 
phyte became  more  profoundly  modified  by  the  stimuli  of  external 
conditions  than  did  the  gametophyte.  For  example,  we  see  in 
the  sporophyte  of  Anfhoceros  and  of  the  mosses,  a  plant  that  is 
practically  dependent  upon  the  gametophyte  only  for  water  and 
the  earth  substances.  So  the  way  is  prepared  for  a  study  of 
the  next  division  where  the  sporophyte  finally  comes  to  absorb 
its  crude  materials  directly  from  the  soil  and  thus  becomes  an 
independent  self-supporting  plant. 


CHAPTER   VIII 

DIVISION   III.     PTERIDOPHYTA  OR  FERNS 

in.  Nature  of  the  Ferns. — The  members  of  this  division  differ 
greatly  in  character  and  live  under  a  wide  range  of  conditions, 
although,  like  the  Bryophyta,  they  usually  frequent  moist  and 
shady  places,  as  glens,  ravines  and  damp  woods,  or  in  tropical 
countries  they  often  cover  the  moist  rocks  and  tree  trunks  with 
a  luxuriant  and  attractive  vegetation.  The  most  noticeable  fea- 


FIG.  213.  The  Christmas  fern,  Polystichum,  with  young  leaves  uncoiling 
in  the  spring  at  the  tip  of  the  stem  and  further  back  the  fully  developed 
leaves  of  the  past  season.  The  older  portion  of  the  stem  is  covered  with 
the  petioles  of  leaves  that  have  died  off  and  numerous  roots  are  seen  arising 
from  the  stem  and  bases  of  the  petioles. — H.  O.  Hanson. 


312  SUPREMACY  OF   THE   SPOROPHYTE 

true  in  the  fern  is  the  large  development  and  complex  character 
of  the  plant  (Fig.  213).  This  is  due  to  a  more  efficient  absorb- 
ing and  conducting  apparatus,  the  root  and  vascular  bundles. 
This  conducting  system  was  foreshadowed  in  the  elongated  cells 
that  appear  in  the  sporophyte  of  Anthoceros  and  of  the  mosses. 
Here  the  elongated  cells  are  much  more  specialized  and  are  united 
into  large  bundles,  termed  the  vascular  bundles,  that  appear  in 
the  leaves  as  "veins"  and  in  the  stems  they  are  variously  related 
and  often  constitute  a  conspicuous  portion  of  these  organs,  as 
is  the  case  in  the  higher  plants,  see  page  76.  The  fern  plant  does 
not  correspond  to  the  moss  plant.  It  is  the  sporophyte  which  has 
become  independent  of  the  gametophyte.  The  sexual  generation 
is  a  small  thallus,  commonly  called  the  prothallium,  that  is  quite 
as  primitive  as  the  simplest  Jungermaniales.  Its  chief  function  is 
the  formation  of  gametes  (page  282)  and  it  is  capable  of  nourish- 
ing the  sporophyte  only  for  a  short  time  in  the  majority  of  ferns 


FIG.  214.  The  thalloid  gametophyte  or  prothallium,  p,  of  a  fern  from 
which  a  young  fern  plant  is  being  developed.  This  sporophyte  has  already 
developed  two  roots,  r,  that  penetrate  the  soil,  and  two  leaves,  /,  are  ap- 
pearing, thus  making  the  fern  at  an  early  age  independent  of  the  gameto- 
phyte; rh,  rhizoids. 

(Fig.  214).  The  gametophyte  of  the  Bryophyta  never  attained 
any  considerable  proportions  owing  to  its  inability  to  vary  and 
produce  adequate  absorbing  organs  and  conducting  tissues. 
This  naturally  limited  the  development  of  the  sporophyte  which 


DEVELOPMENT  OF   PLANTS  313 

was  parasitic  upon  it.  In  the  Pteridophyta,  however,  the 
sporophyte  is  not  at  such  a  disadvantage  since  it  actually  becomes 
independent  of  the  gametophyte  owing  to  the  development  of  a 
root  which  puts  it  in  communication  with  the  earth  substances. 
This  radical  departure  in  its  mode  of  life  acted  as  a  profound 
stimulus  and  cooperated  in  inducing  marked  variations  that  were 
so  beneficial  in  character  as  to  cause  the  sporophyte  to  assume 
large  proportions  and  become  differentiated  into  a  highly  or- 
ganized plant.  The  stem  elongates  through  the  repeated  divi- 
sion of  a  single  apical  cell,  as  in  the  Bryophyta,  but  the  stem  and 
also  the  leaves  and  roots  in  addition  contain  vascular  bundles 
and  an  arrangement  of  tissues  already  noticed  in  the  higher 
plants. 

The  gametophyte  in  many  of  the  Pteridophyta  is  very  sugges- 
tive of  the  simpler  thalloid  hepatics.  Owing  to  the  fact  that  it 
is  no  longer  permanently  burdened  with  the  nutrition  of  the 
sporophyte,  it  becomes  greatly  reduced  in  size  and  length  of  life. 
In  fact,  in  several  of  the  more  specialized  ferns,  the  gameto- 
phyte may  be  reduced  to  a  few  cells  and  the  entire  development 
may  take  place  within  a  day.  The  reproductive  organs  and  the 
germination  of  the  gametospore  are  suggestive  of  the  correspond- 
ing features  noted  in  the  Bryophyta.  The  more  important  orders 
of  the  Pteridophyta  are:  I.  Ophioglossales.  2.  Filicales  or  Com- 
mon Ferns.  3.  Equisetales  or  Horsetails.  4.  Lycopodiales  or 
Club  Mosses.  This  sequence  is  followed  because  it  brings  out 
more  clearly  the  trends  or  tendencies  that  arose  step  by  step  in 
the  evolution  of  plant  life.  As  a  matter  of  fact  the  club  mosses 
appear  to  be  the  most  primitive  forms  and  they  are  the  most 
suggestive  of  the  Bryophyta;  while  the  common  ferns  are  the 
most  recent  in  development.  Enough  has  been  said  to  show  that 
these  forms  are  very  remotely  related  to  the  ancestors  of  the  Bry- 
ophyta and  that  these  four  orders  of  Pteridophyta  are  very  dis- 
tantly connected. 

Order  i.     Ophioglossales 

112.  General  Characters  of  the  Sporophyte. — This  order  con- 
tains three  genera  which  are  probably  but  a  remnant  of  an 
earlier  and  widely  distributed  group.  Only  two,  Ophioglossum 

21 


3H  STRUCTURE   OF   OPHIOGLOSSALES 

and  Botryckium,  are  of  common  occurrence  (Fig.  215).  They 
are  of  unusual  interest  because  they  present  many  features  sug- 
gestive of  the  liverworts  and  also  of  the  more  specialized  ferns 
and  seed  plants.  This  remark  is  not  intended  as  necessarily 
implying  relationship  between  the  three  groups,  but  that  analo- 
gous structures  have  arisen  in  each,  due  either  to  inheritance 
from  allied  ancestors  or  to  the  operation  of  stimuli  upon  plants 
that  are  only  slightly  or  not  at  all  related.  The  sporophyte 
consists  of  a  short  stem  with  thick  fleshy  roots  that  are  associated 
with  mycorrhiza.  The  leaves  are  simple  or  divided  and  usually 
appear  singly,  ensheathing  the  apex  of  the  stem  which  always 
remains  in  the  soil.  One  of  the  most  remarkable  features  about 
these  ferns  is  the  development  in  the  stems,  roots  and  leaves 
of  the  same  tissues  that  we  have  noted  in  the  higher  seed  plants. 
The  vascular  bundles  have  the  same  origin  as  in  the  seed  plants. 
The  woody  portion  (xylem)  of  the  conducting  system  appears  as 
a  solid  strand  in  the  center  of  the  very  young  stem  and  this  is 
surrounded  by  the  cellulose  part  (phloem)  of  the  system.  Early 
in  the  development  of  the  stem  delicate  cells  (pith)  replace  the 
central  cells  of  the  xylem  so  that  the  conducting  system  now 
appears  as  a  hollow  cylinder.  As  the  stem  elongates  this  cylinder 
is  broken  into  segments  by  certain  strands  from  it  passing  out 
into  the  leaves.  The  spaces  formed  by  the  departure  of  these 
strands  are  termed  foliar  gaps.  The  segments  of  the  con- 
ducting system  are  termed  the  vascular  bundles  and  since  the 
phloem  is  outside  the  xylem  they  are  known  as  collateral  bundles. 
In  some  cases  these  bundles  develop  a  cambium  which  adds  new 
cells  to  the  xylem  and  phloem  so  that  the  arrangement  of  tissues 
in  the  stem  is  strikingly  suggestive  of  the  dicotyledonous  stem 
with  its  pith;  concentric  rings  of  xylem,  cambium  and  phloem; 
rays;  cortex;  cork;  etc.  (Fig.  216,  40).  It  is  noteworthy  also 
that  the  cells  added  to  the  xylem  by  the  cambium  resemble  those 
appearing  in  the  pines.  The  stem  structure  of  some  of  the 
simpler  representatives  of  the  next  order  resembles  that  of  the 
Ophioglossales  and  it  has  been  suggested  that  the  ancestors  of 
these  two  lines  of  P,teridophyta  may  have  been  the  starting  point 
of  the  seed  plants. 


DEVELOPMENT   OF   PLANTS 


315 


The  spores  are  borne  in  peculiar  modified  outgrowths  of  the 
leaves  that  assume  a  cylindrical  shape  in  Ophioglossum  and  be- 
come more  or  less  branched  in  the  species  of  the  Botrychium 
(Fig.  215).  The  structure  of  these  organs  presents  many  fea- 
tures that  recall  the  sporophyte  of  Anthoceros  (Figs.  215,  B\ 
199).  The  outer  portion  of  the  cylinder  consists  of  chlorophyll- 
bearing  cells  which  communicate  with  stomata  in  some  of  the 
forms  at  least.  The  spore  mother  cells  are  also  distributed  as 


FIG.  215.  The  two  common  genera  of  the  Ophioglossales:  A,  the  adder- 
tongue  fern,  Ophioglossum,  with  single  leaf  ensheathing  the  short  stem  and 
producing  a  spore-bearing  spike.  B,  section  of  the  spike — sp,  the  spore- 
forming  cells  arranged  in  groups  or  sporangia.  The  spores  are  exposed  by 
the  breaking  apart  of  the  cells  between  the  dark  lines.  C,  stoma  from  epi- 
dermis of  spike.  D,  the  grape  fern,  Botrychium.  The  leaf  is  much  divided 
and  also  forms  a  branched  spore-bearing  organ.  E,  two  sporangia,  showing 
the  manner  of  opening  for  discharge  of  spores. 

in  Anthoceros,  the  essential  difference  being  that  they  are  sepa- 
rated by  a  larger  number  of  sterile  cells  and  a  larger  number  of 
spore  mother  cells  are  also  grouped  together,  forming  rather  con- 
spicuous sacs  or  sporangia  (Fig.  215,  sp).  The  spores  are  formed 


316 


NATURE   OF    THE   SPORANGIA 


from  the  mother  cells  as  in  the  Bryophyta  and  are  discharged 
from  the  sporangia  through  a  transverse  cleft  (Fig.  215,  £).  It 
is  interesting  to  note  in  a  closely  allied  order  occurring  in  the 


FIG.   216.     Cross-section  of  a   stem   of  Botrychium:  p,   pith;   x,   xylem; 
m,  ray;  c,  cambium;  ph,  phloem;  e,  endodermis;  cr,  cortex. — After  Jeffrey. 

tropics,  that  the  sporangia  are  developed  directly  upon  the  leaves 
(Fig.  217)  instead  of  upon  special  branches  and  that  they  are 
sometimes  associated  together  in  groups,  known  as  sori  (sing. 


FIG.  217.  Arrangement  of  the  sporangia  of  an  allied  order,  Marattiales; 
A,  leaflet  of  Archangiopteris  with  sporangia  on  surface  of  leaf  and  arranged 
in  groups  or  sori.  B,  magnified  view  of  a  portion  of  the  leaflet.  C,  section 
of  leaf,  showing  two  sporangia,  the  left-hand  one  in  section. 

sorus)  and  provided  with  thickened  cells,  the  annulus — charac- 
ters that  will  become  conspicuous  in  the  next  order.  These 
sporangia  originate,  however,  as  in  Ophioglossum  and  open  by 
transverse  clefts,  which  operation  is  promoted  by  the  unequal 
drying  of  the  thick-  and  thin-walled  cells  of  the  annulate  forms, 
(a)  The  Gametophyte. — The  gametophyte  or  sexual  genera- 


DEVELOPMENT  OF   PLANTS 


317 


tion  is  not  often  met  with,  owing  to  the  fact  that,  like  the  sporo- 
phyte,  its  development  is  associated  with  mycorrhizal  fungi  and 
consequently  it  is  usually  completely  buried  in  the  soil  and  usually 
quite  devoid  of  chlorophyll.  The  various  stages  in  the  germina- 
tion of  the  spores  have  never  been  observed,  but  the  mature  gam- 
etophyte  of  Botrychium  was  found  a  few  years  ago  by  Jeffrey 


FIG.  218.  The  gametophyte  and  young  sporophyte  of  Botrychium:  Ay 
tuberous  appearance  of  the  gametophyte — e,  a  young  sporophyte  or  em- 
bryo developing  in  one  of  the  archegonia.  B,  a  section  of  gametophyte — 
ar,  archegonia;  an,  antheridia.  C,  two  antheridia  in  section.  D,  male 
gamete.  E,  archegonium  before  dissolution  of  canal  cells.  F,  gametospore 
in  two-cell  stage  of  germination.  G,  young  sporophyte  with  roots  and  first 
leaf  developed  but  still  attached  by  foot  to  the  round  gametophyte,  gm. 
A.  F.— After  Jeffrey. 

and  carefully  studied.  It  appeared  as  a  rather  tuberous  body 
that  is  provided  with  numerous  rhizoids  and  a  growing  apical 
cell  as  in  the  Bryophyta  (Fig.  218,  A,  B}.  The  archegonia  and 
antheridia  are  usually  borne  upon  the  upper  surface  of  the  game- 


3i8      NATURE  OF  THE  GAMETOPHYTE 

\ 
tophyte  and  in  origin  and  appearance  are  rather  suggestive  of 

Anthoceros.  The  antheridia  consist  of  a  number  of  mother  cells 
quite  buried  in  the  tissues  of  the  thallus,  being  covered  by  one 
or  two  layers  of  cells  which  are  destroyed  or  break  open  when 
the  male  gametes  are  mature  (Fig.  218,  C).  These  gametes  are 
quite  different  from  any  yet  seen,  being  spirally  coiled  and  pro- 
vided with  numerous  cilia  (Fig.  218,  D).  The  archegonia  are 
similar  to  those  noted  in  Anthoceros,  consisting  of  a  rather  short 
neck  with. two  central  canal  cells  and  a  swollen  base  sunken  in 
the  tissues  of  the  gametophyte  (Fig.  2 1 8,  E) .  The  female  gamete 
is  formed  at  the  base  of  the  canal  cells  which  finally  become 
mucilaginous  and  thus  form  an  open  passage-way  for  the  entrance 
of  the  male  gametes  as  soon  as  the  lip  cells  have  opened.  It  has 
been  shown  in  the  common  ferns  that  malic  acid  is  present  in 
this  mucilaginous  substance  which  strongly  attracts  the  male 
gametes  so  that  they  crowd  into  the  canal,  often  completely  chok- 
ing it.  So  we  see  that  the  gametophyte  of  this  fern  (though 
differing  in  form)  is  essentially  of  the  same  character  as  in  the 
Bryophyta,  its  subterranean  character  and  absence  of  chlorophyll 
being  due  to  its  association  with  fungi.  It  should  be  stated  that 
lobes  are  produced  from  the  end  of  the  tuberous  gametophyte 
of  one  of  the  species  which  become  green  on  reaching  the  surface 
of  the  soil  and  in  a  closely  allied  order  or  tropical  ferns,  the 
sexual  generation  is  normally  a  green  thallus  strikingly  like  some 
of  the  simpler  Jungermaniales. 

(b)  The  Germination  of  the  Gametospore. — The  early  develop- 
ment of  the  sporophyte  resembles  Anthoceros  in  many  respects. 
The  gametospore  divides  into  two  cells  (Fig.  218,  F)j  from  the 
lower  of  which  a  large  foot  is  formed,  and  from  the  upper  cell 
the  short  stem  and  root  arise.  Later,  the  first  leaf  or  cotyledon 
is  formed  from  the  stem.  This  development  of  the  sporophyte 
goes  on  very  slowly  within  the  archegonium  where  it  lives  as  a 
parasite  for  a  long  time.  Eventually  the  root  ruptures  the  arche- 
gonium or  calyptra  and  comes  in  contact  with  the  soil.  The 
cotyledon  now  grows  upward  and  makes  its  appearance  above 
the  ground  as  the  first  green  leaf  (Fig.  218,  G).  The  sporophyte 
thus  becomes  a  self-supporting  plant,  although  it  probably  re- 


DEVELOPMENT   OF   PLANTS  319 

mains  in  part  dependent  upon  the  gametophyte  for  several  years. 
At  this  stage  of  development  the  young  sporophyte  is  a  very 
simple  type  of  fern,  but  gradually  more  efficient  roots  are  de- 
veloped and  each  year  the  stem  sends  up  a  larger  and  larger  leaf 
until  the  adult  size  is  reached  when  the  spore-bearing  branch  is 
formed  (Fig.  215),  thus  making  possible  again  a  new  series  of 
gametophytes. 

(c)  Comparison  of  the  Adder  Tongue  Ferns  with  Preceding 
Groups. — The  most  noteworthy  difference  between  these  simple 
ferns  and  the  Bryophyta  is  the  larger  development  of  the  sporo- 
phyte and  its  final  independence  of  the  gametophyte.  This  is 
doubtless  due  to  the  development  of  true  roots  which  made  pos- 
sible a  continuous  and  abundant  supply  of  the  crude  materials 
from  the  soil.  This  change  acted  as  a  stimulus  which  promoted 
variations  in  the  sporophyte  while  the  light  and  various  climatic 
factors  also  assisted  to  a  very  marked  degree.  Among  the  algae, 
the  gametophyte  is  the  dominant  generation,  the  sporophyte 
being  represented  often  by  the  gametospore.  This  is  essentially 
the  relationship  among  the  majority  of  the  Hepaticae  where  the 
gametophyte  performs  all  the  work  of  food  construction  and  the 
sporophyte  is  a  minute  parasitic  plant  upon  it.  In  Anthoceros 
and  the  mosses,  the  sporophyte  is  longer  lived  and  more  highly 
organized,  but  the  gametophyte  is  still  the  more  important  gen- 
eration as  it  performs  the  major  portion  of  the  work.  Among  the 
ferns,  this  relationship  and  the  distribution  of  labor  is  completely 
turned  about.  The  sporophyte  becomes  the  larger  plant  and  the 
principal  center  of  photosynthesis,  while  the  gametophyte  re- 
mains small  and  gradually  becomes  very  short  lived. 

There  is  also  to  be  noted  the  longer  postponement  in  the  for- 
mation of  the  spores.  In  certain  of  the  algae  the  gametospores 
produce  the  spores  directly  on  germinating  while  in  Ricciocarpus 
a  small  mass  of  cells  is  first  formed  and  certain  of  these  cells 
soon  become  spore  mother  cells.  In  Marchantia,  a  larger  number 
of  cells  are  formed  by  the  gametospore  and  the  spore  mother  cells 
originate  after  a  few  weeks  in  a  definite  region  of  the  sporophyte. 
These  features  are  more  noticeable  in  the  Jungermaniales  and 
especially  in  the  long-lived  sporophyte  of  Anthoceros.  Recall 


320  DECLINE   OF  THE  GAMETOPHYTE 

also  the  considerable  growth  of  the  sporophyte  of  the  Bryales 
and  the  final  development  of  the  spore  mother  cells  in  a  definite 
and  restricted  region  in  the  capsule.  In  the  Pteridophyta,  the 
sporophyte  not  only  becomes  larger  and  more  highly  differenti- 
ated, but  the  formation  of  the  spores  is  often  deferred  for  years 
and  the  spore  mother  cells  are  localized  in  special  organs.  This 
postponement  of  the  spore  formation  may  appear  at  first  sight 
as  a  disadvantage  since  the  sporophyte  is  exposed  to  many  dan- 
gers during  the  long  period  of  preparation  for  its  work.  However, 
the  delay  is  more  than  compensated  for  by  the  large  number  of 
spores  that  are  finally  formed  and  also  by  the  fact  that  the  sporo- 
phyte does  not  entirely  perish  as  in  previous  cases,  but  lives  on, 
sending  up  annually  new  spore-bearing  leaves.  It  would  appear 
that  a  point  had  at  last  been  reached  in  the  evolution  of  the 
sporophyte  where  it  is  so  well  organized  as  to  ensure  its  exist- 
ence. It  can  consequently  with  safety  defer  the  formation  of 
spores  until  well-developed  roots,  stems  and  leaves  have  been 
formed.  The  independent  existence  of  the  sporophyte  brings 
out  very  clearly  the  two  phases  in  the  life  history  of  the  plant 
which  we  call  the  alternation  of  generations.  The  formation  of 
the  spores  marks  the  beginning  of  the  gametophyte  or  sexual 
generation,  which  ends  with  the  formation  of  the  gametes.  The 
formation  of  the  gametospore  through  the  fusion  of  the  gametes 
starts  the  sporophyte  or  asexual  generation  which  ends  with  the 
division  of  the  spore  mother  cells. 

Why  has  not  the  gametophyte  varied  as  well  as  the  sporophyte? 
Perhaps  the  simplicity  of  its  structure  has  become  fixed  by  in- 
heritance from  a  long  line  of  algal  ancestors.  The  occurrence 
in  the  Bryophyta  and  Pteridophyta  of  motile  male  gametes  and 
the  consequent  necessity  of  water  for  fertilization,  points  to  the 
inheritance  of  this  peculiarity  from  aquatic  ancestors.  It  is  evi- 
dent that  there  can  be  no  considerable  modification  of  the  gameto- 
phyte, as  long  as  this  feature  is  retained,  without  greatly  decreas- 
ing the  possibilities  of  fertilization.  Consequently  the  gameto- 
phyte must  remain  of  necessity  a  primitive  structure.  The 
sporophyte,  on  the  other  hand,  was  accustomed  from  the  first  to 
terrestrial  conditions,  or  to  such  conditions  as  were  unfavorable 


DEVELOPMENT  OF   PLANTS  321 

to  the  growth  of  the  gametophyte.  Among  the  algae  we  have 
noticed  that  it  may  carry  the  plant  over  the  more  or  less  com- 
plete drying  up  of  the  water  or  unfavorable  temperatures.  Per- 
haps these  exposures  to  a  variety  of  stimuli  which  the  gameto- 
phyte never  experienced  or  in  a  lesser  degree  enabled  the  sporo- 
phyte  to  respond  with  more  profound  variations  when  it  became 
parasitic  upon  the  gametophyte  and  exposed  directly  to  the 
light  and  air  as  was  the  case  among  the  Bryophyta.  Certainly 
we  know  that  abundance  of  foods,  light,  temperatures,  etc., 
are  among  the  important  stimuli  in  causing  variations.  The 
simple  sporophyte  of  the  Bryophyta  was  exposed  to  just  such 
forces  as  these  and  steadily  gained  in  complexity  until  in  the 
Bryales  it  nearly  equaled  in  importance  the  gametophyte.  The 
sporophyte  of  the  Pteridophyta,  owing  to  the  development  of  the 
root,  experiences  a  new  stimulus,  i.  e.,  that  of  the  soil  which,  co- 
operating with  the  stimuli  of  the  light,  etc.,  causes  it  to  assume 
much  larger  proportions  than  the  gametophyte  and  become  the 
dominant  generation  in  the  life  history  of  the  plant.  Perhaps 
another  factor  played  an  important  role  in  securing  the  supremacy 
of  the  sporophyte.  We  have  seen  in  paragraph  58,  The  Signifi- 
cance of  Fertilization,  that  the  sporophyte  is  the  result  of  the 
fusion  of  two  gametes  which  may  vary  in  character.  The  con- 
stant introduction  of  variant  characters  into  the  asexual  genera- 
tion in  this  way  may  have  cooperated  in  its  increasing  complexity. 

Order  2.    Filicales  or  Common  Ferns 

113.  General  Features. — The  great  majority  of  plants  popu- 
larly known  as  ferns  belong  to  this  order.  They  are  a  highly 
specialized  group  that  have  branched  off  from  some  primitive 
fern  stock  in  recent  geological  times  and  owing  to  their  variations 
being^  highly  adapted  to  present  conditions  upon  the  earth,  they 
have  become  very  numerous  and  widely  distributed — probably 
exceeding  3000  species.  In  temperate  climates  the  majority 
live  upon  the  ground  in  moist  and  shady  regions  and  some  are 
aquatic  or  xerophytic.  They  attain  their  greatest  abundance 
in  the  mountainous  district  of  tropical  countries  where  they  occur 
in  astonishing  profusion  and  variety  upon  the  moist  rocks  and 


322 


LEAVES   OF   FILICALES 


trunks  of  trees,  as  well  as  upon  the  earth.  The  leaves  are  the 
most  striking  feature  of  the  Filicales.  They  are  usually  large  and 
divided  and  are  characterized  by  being  coiled  when  young  (Fig. 
219).  This  is  due  to  the  stronger  growth  of  the  outer  cells  which 
causes  an  inward  rolling  of  the  leaf.  The  growth  of  the  leaf  is 
very  slow,  often  requiring  three  years  for  its  formation  in  tem- 


FlG.  219.  Christmas  fern,  Polystichum,  with  prostrate  stem  bearing  at 
the  tip  young  coiled  leaves  covered  with  chaffy  scales  and  further  back  large 
leaves  of  the  previous  season.  The  older  portions  of  the  stem  are  covered 
with  the  petioles  of  the  dead  leaves. — H.  O.  Hanson. 

perate  regions.  During  the  season  preceding  its  expansion,  the 
petiole  and  blade  are  completely  formed  and  appear  in  crosier- 
like  coils  more  or  less  covered  with  chaffy  scales  (Fig.  220,  c). 
This  development  enables  the  leaves  to  expand  with  surprising 
rapidity  in  the  spring  when  the  more  rapid  enlargement  of  the 
cells  on  the  inner  side  of  the  leaf  cause  it  to  uncoil.  There  is 


DEVELOPMENT   OF   PLANTS 


323 


another  feature  about  these  leaves  that  compels  our  admiration. 
Comparatively  few  leaves  are  produced  annually  from  the  tip 
of  each  stem — in  the  preceding  order  usually  but  one.  This 


FIG.  220.  Stem  in  early  spring  freed  from  all  of  its  leaves  save  the  young 
ones,  c,  near  the  tip:  r,  roots;  v,  vascular  bundles  in  base  of  petiole;  x,  region 
from  which  the  cortex  has  been  removed  to  show  the  vascular  bundles  from 
the  leaves  uniting  to  form  a  lattice  work. 

appears  to  be  the  rule  in  underground  stems,  since  the  difficulty 
and  danger  of  sending  the  tender  young  leaves  up  through  the 
soil  is  minimized  by  the  development  of  one  or  a  few  leaves  which 


FIG.  221.  Cross-section  of  fern  stem:  v,  a  concentric  vascular  strand,  the 
large  cells  of  the  xylem  being  surrounded  by  the  phloem.  Each  strand  is 
surrounded  by  a  compact  layer  of  cells,  the  endodermis;  st,  stereome;  cr, 
cortical  region. 

form  large  blades  after  reaching  the  air.  In  the  two  remaining 
orders  of  Pteridophyta,  branches  of  the  underground  stems  rise 
above  the  ground  and  these  bear  numerous  small  leaves,  also  a 
rule  for  stems  of  this  kind.  The  tissues  of  the  leaves  are  differ- 


324  STRUCTURE  OF  THE  STEM 

entiated  into  an  epidermis,  stomata,  chlorenchyma  and  vascular 
bundles  as  in  the  higher  plants  (Fig.  223).  The  stems  are  more 
usually  prostrate,  creeping  rhizomes  that  branch  sparingly  and 
so  gradually  give  rise  to  colonies  of  ferns.  One  of  the  most 
attractive  features  of  certain  tropical  districts  is  the  tree  ferns 
with  erect  stems  of  palm-like  appearance  which  lift  their  great 
crowns  of  leaves  30  to  50  feet  in  the  air.  The  vascular  bundles 
are  more  commonly  of  the  concentric  type  (Fig.  221).  Perhaps 
we  should  repeat  that  a  bundle  is  composed  of  two  types  of  elon- 
gated cells,  thick  walled  woody  cells  (the  xylem)  and  thin  walled 


FIG.  222.  Arrangements  of  the  sporangia:  A,  lobe  of  leaf  of  Dryopteris 
with  sporangia  grouped  in  circular  sori,  s.  B,  sorus  enlarged,  showing  the 
shield-like  membrane  or  indusium,  in,  covering  the  sporangia,  sp.  C,  lobe 
of  leaf  of  Asplenium  with  elongated  sori,  5. 

cellulose  cells  (the  phloem).  In  the  fern  type  of  concentric 
bundle  the  phloem  surrounds  the  xylem.  These  bundles  are 
united  in  the  stem  in  a  variety  of  ways.  In  the  simplest  case 
we  find  a  central  core  of  xylem  surrounded  by  phloem  which 
gives  off  strands  or  vascular  bundles  to  the  leaves  and  roots.  In 
the  majority  of  ferns  this  core  of  xylem  is  modified  by  the  develop- 
ment of  rather  delicate  pith-like  cells  in  its  center,  consequently 
it  appears  as  a  ring  in  cross  section  or  as  a  cylinder  in  longitudinal 
view.  This  ring  is  broken  into  segments  owing  to  the  fact  that 


DEVELOPMENT   OF   PLANTS  325 

portions  of  it  turn  out  here  and  there,  thus  forming  the  vascular 
.bundles  that  extend  into  the  leaves.  Such  an  arrangement  is 
seen  in  Botrychium  (Fig.  216).  These  spaces  formed  in  the  ring, 
called  foliar  gaps,  are  often  quite  large  so  that  the  cylinder  of 
vascular  tissue  in  the  stem  presents  the  appearance  of  a  lattice 
work — note  the  vascular  tissue  at  the  right-hand  end  of  the  stem 
in  Fig.  220.  In  cross  section  such  a  cylinder  would  appear  as  a 
series  of  more  or  less  widely  separated  strands  (Fig.  221).  This 
latter  figure  shows  that  strands  arising  from  the  lattice-like 
cylinder  may  also  extend  into  the  pith-like  region — note  the  three 
large  strands  in  Fig.  221,  v.  This  explains  the  confusing 
arrangement  of  strands  often  seen  in  the  cross  section  of  fern 
stems  and  previously  noted  in  monocotyledons,  p.  96.  Certain 
cells  of  the  cortex  and  pith  often  become  modified  into  strengthen- 
ing cells  of  stereome  (Fig.  221,  sf).  Roots  arise  near  the  base 
of  the  leaves,  and  in  some  of  the  tree  ferns  form  a  thick  mat-like 
covering  on  the  stems.  They  originate  from  the  endodermis 
of  the  bundles  and  possess  a  root  cap  and  radial  arrangement  of 
the  vascular  bundles  as  in  higher  plants. 

(a)  Structure  and  Character  of  the  Sporangia. — The  sporangia, 
instead  of  being  produced  in  the  tissues  of  special  branches  as 
in  Ophioglossum,  are  borne  in  curiously  constructed  capsules, 
usually  situated  on  the  under  surface  of  the  ordinary  green 
leaves  (Fig.  222).  The  sporangia-bearing  leaves  are  usually 
called  sporophylls,  meaning  spore-bearing  leaves.  In  some  cases 
the  sporophylls  are  highly  modified,  being  entirely  given  up  to 
spore  production  and  therefore  quite  different  from  the  green 
leaves  (Fig.  227).  The  sporangia  are  usually  associated  in 
groups  or  sori  (sing,  sorus)  on  the  vascular  bundles  and  pro- 
tected by  a  membranous  outgrowth  of  the  epidermis,  known 
as  the  indusium  (Fig.  222,  B).  Each  sporangium  originates 
usually  from  a  single  epidermal  cell,  which  by  repeated  divisions 
(Fig.  223)  produces  a  capsule  or  sporangium  that  contains  the 
spores.  A  sporangium  of  the  shield  fern  contains  48  spores  and 
there  are  fully  100  sporangia  to  a  sorus  and  20  sori  on  each  lobe 
or  pinna  of  the  leaf.  A  well-developed  leaf  would  have  on  an 
average  50  pinnae  and  a  healthy  plant  would  bear  about  10 


326  SPORANGIA   OF   FILICALES 

leaves.  Therefore,  Bower  estimates  that  the  shield  fern  produces 
annually  upwards  of  50  million  spores.  This  makes  a  striking 
comparison  when  we  consider  the  spore  production  in  the  Bryo- 


FIG.  223.  Section  of  a  leaf  of  Woodwardia,  showing  two  sori:  i,  indusium; 
sp,  sporangia  arising  from  the  epidermis  and  in  various  stages  of  develop- 
ment. Note  the  epidermis,  stoma,  s,  palisade  and  spongy  chlorenchymal 
and  vascular  bundles,  as  in  higher  plants. 

phyta,  and  it  is  evident  that  ample  provision  is  made  for  the 
maintenance  of  the  race  notwithstanding  the  postponement  and 
specialization  in  spore  production. 

The  sporangia  vary  considerably  in  structure  in  the  various 


FIG.  224.  Character  of  sporangia:  A,  simple  type  of  sporangium  of 
Osmunda  with  rudimentary  annulus,  an,  of  a  few  thickened  cells.  B,  com- 
mon type  of  sporangium — an,  annulus;  /,  lip  cells. 

genera.  In  the  simplest  forms,  they  consist  of  a  uniform  layer  of 
wall  cells  inclosing  a  mass  of  cells,  the  majority  of  which  become 
spore  mother  cells,  producing  four  spores  each.  In  other  cases, 
a  few  of  the  wall  cells  are  thickened,  forming  a  rudimentary 


DEVELOPMENT   OF   PLANTS  327 

annulus  (Fig.  224,  A),  but  in  the  majority  of  our  common  ferns, 
the  sporangium  consists  of  a  delicate  stalk,  supporting  a  rather 
spherical  spore-bearing  capsule  (Fig.  224,  B).  The  walls  of  this 
capsule  consist  of  a  single  layer  of  thin-walled  cells  except  for  a 
row  of  thickened  cells,  the  annulus,  that  extends  from  the  stalk 
over  the  capsule  nearly  to  the  opposite  side  (Fig.  224,  By  an). 
At  the  latter  point,  the  annulus  ends  in  a  few  rather  weakened 
cells,  two  of  which,  the  lip  cells,  are  conspicuous  for  their  larger 
size  (Fig.  224,  B,  I).  It  is  to  be  noticed  that  the  cells  of  the  an- 
nulus are  thickened  on  their  inner  and  radial  walls  while  the  outer 
walls  remain  comparatively  thin.  When  the  spores  are  mature 
and  lie  loose  in  the  capsule,  the  cells  of  the  annulus  that  have 
been  filled  with  water  up  to  this  time  begin  to  dry  out.  As  the 
volume  of  water  in  these  cells  lessens  through  evaporation,  the 
cohesion  of  the  water  pulls  upon  the  thin  outer  walls,  causing 
them  to  bend  in,  assuming  a  U-shaped  appearance.  This  tends 
to  pull  the  two  radial  walls  of  each  cell  together  and  shorten 
the  outer  circumference  of  the  annulus.  This  contraction  pulls 
open  the  sporangium  at  the  weak  lip  cells,  the  split  extending 
back  through  the  lateral  cells  of  the  capsule  which  is  finally 
drawn  back  to  such  an  extent  that  the  annulus  becomes  nearly 
straight.  Finally,  when  the  water  is  nearly  withdrawn  from  the 
cells,  the  tension  becomes  so  great  that  air  is  drawn  in  through 
the  thin  walls  of  the  cells.  The  entrance  of  air  breaks  the  water 
adhering  to  the  walls  and  which  had  kept  them  pulled  in  as 
described  above  and  consequently  each  cell  returns  instantly  to 
its  original  form.  This  causes  the  capsule  to  snap  back  to  near 
its  original  position  with  a  quick  jerk,  thus  throwing  out  the 
spores  to  a  considerable  distance.  This  motion  of  the  capsule 
can  be  approximated  by  mounting  sporangia  that  have  been 
soaked  in  a  drop  of  water  and  watching  them  under  a  micro- 
scope while  a  drop  of  glycerine  placed  in  touch  with  one  side 
of  the  mount  is  drawn  in  with  a  filter  paper  applied  to  the  oppo- 
site side.  The  denser  glycerine  will  draw  the  water  out  of  the 
cells  by  osmosis  and  set  up  a  tension  in  the  cells  of  the  annulus 
just  as  did  the  evaporation  of  the  water. 

The  form  of  the  sorus  and  its  relation  to  the  indusium  and 


328 


FORMS   OF   FILICALES 


particularly  the  structure  of  the  annulus  are  important  charac- 
teristics to  be  observed  in  identifying  the  ferns.  In  the  flowering 
ferns,  Osmunda,  the  large  slight-stalked  sporangia  are  pro- 
vided with  a  rudimentary  annulus  of  a  few  thickened  cells  lo- 
cated at  one  side  of  the  capsule  (Fig.  224,  A).  The  entire  leaf 
or  certain  leaflets  are  completely  covered  with  the  sporangia, 
little  more  than  the  vascular  bundles  remaining  (Fig.  225,  A). 


FIG.  225.  Common  forms  of  the  Filicales:  A,  the  flowering  fern,  Os- 
munda, showing  below  two  green  leaflets  and  above  two  sporangia-bearing 
leaflets.  At  left  a  cluster  of  sporangia  magnified.  The  first  leaves  in  the 
spring  only  bear  sporangia;  those  appearing  later  have  only  green  leaves. 
B,  chain  fern,  Woodwardia.  C,  Christmas  fern,  Polystichum.  D,  bladder 
fern,  Filix.  E,  hay-scented  fern,  Dennstaedtia — s,  sorus  enlarged.  F,  Woodsia 
— After  Sprague. 

Osmunda  belongs  to  the  lowest  group  of  the  Filicales  and  shows 
certain  analogies  in  the  arrangement  and  structure  of  its  collateral 
vascular  bundles  with  the  Ophioglossales.  The  majority  of 
ferns  in  temperate  climates  are  characterized  by  sporangia  of 
the  type  shown  in  Fig.  224,  B,  and  they  are  distinguished  by 
the  form  and  arrangement  of  the  sori  and  indusia.  The  shield 
fern,  Dryopteris,  has  a  rather  circular  or  curved  sorus  covered 


DEVELOPMENT   OF   PLANTS 


329 


by  an  indusium  that  is  laterally  attached  at  a  single  point  (Fig. 
222,  A).  The  Christmas  fern,  Polystichum,  has  a  circular  sorus 
with  centrally  attached  indusium  (Fig.  225,  C).  The  spleenwort 
fern,  Asplenium,  is  characterized  by  elongated  sori  arranged 
obliquely  to  the  midrib  upon  the  upper  side  of  the  veinlets. 
The  indusium  is  attached  on  one  side  of  the  sorus  along  its  entire 
length  (Fig.  222,  B).  The  chain  fern,  Woodwardia,  differs 
from  Asplenium  in  having  the  sori  arranged  in  chain-like  rows 


FIG.  226.  Ferns  without  indusia  or  possessing  false  ones:  A,  leaflet  of 
bracken  fern,  Pteridium.  B,  maiden-hair  fern,  Adiantum.  C,  polypod  fern, 
Polypodium. — After  Sprague. 

parallel  to  its  midrib  (Fig.  225,  B).  In  several  genera  of  ferns 
the  indusium  is  partly  or  entirely  inferior.  Thus  in  the  bladder 
fern,  Filix,  the  partly  inferior  indusium  covers  the  circular  sorus 
like  a  hood  (Fig.  225,  D),  while  it  is  wholly  inferior  in  the  hay- 
scented  fern,  Dennstaedtia,  forming  a  cup  (Fig.  225,  E)  and  in 
Woodsia  the  indusium  is  roundish  or  star-like  (Fig.  225,  F). 

22 


330  GAMETOPHYTE   OF   FILICALES 

Several  ferns  are  distinguished  by  false  indusia  that  are  formed 
by  the  more  or  less  modified  margins  of  the  leaf.  In  the  bracken, 
Pteridium,  the  entire  membranous  margin  of  the  leaf  curves  over 
the  closely  crowded  sori  (Fig.  226,  A),  and  in  the  maiden-hair 
fern,  Adiantum,  the  sporangia  are  at  the  ends  of  the  veins  and 
covered  by  reflexed  portions  of  the  leaf  (Fig.  226,  B).  The 
indusia  are  lacking  in  some  forms,  as  in  the  beech  fern,  Phegopteris, 
and  in  the  polypody,  Polypodium  (Fig.  226,  C).  These  genera 
are  distinguished  by  the  fact,  among  others,  that  the  leaves  of 
Polypodium  drop  off,  leaving  a  scar  as  in  our  deciduous  trees. 


B 


FIG.  227.  The  sensitive  fern,  Onoclea:  A,  portion  of  normal  green  leaf. 
B,  a  spore-bearing  leaf.  C,  two  views  of  one  of  the  round  lobes  of  B,  showing 
the  veins  and  the  sori  on  inner  side  of  the  lobe. — After  Bailey. 

This  feature  is  possibly  due  to  its  more  or  less  epiphytic  habit. 
This  is  the  only  really  xerophytic  fern  of  temperate  regions. 
The  sporophyll  of  the  sensitive  fern,  Onoclea,  bears  a  striking 
outward  resemblance  to  Botrychium,  but  the  sporangia-like  bodies 
are  really  leaf  lobes  rolled  up  and  each  bears  several  round 
sori  on  its  inner  side  (Fig.  227).  In  this  and  several  other  genera, 
see  Osmunda,  the  work  of  photosynthesis  is  given  over  to  large 
green  leaves  that  do  not  produce  sporangia. 

(b)  The  Gametophyte. — In  the  majority  of  ferns  the  spore 
germinates  by  rupturing  the  outer  coats  and  producing  a  germ 
tube  from  which  one  or  more  delicate  rhizoids  are  cut  off.  The 
germ  tube  elongates,  forming  a  short  chain  of  cells  which  soon 
develop  by  apical  growth  into  a  flat  thalloid  structure,  commonly 
called  the  prothallium,  that  is  attached  to  the  ground  by  numer- 


DEVELOPMENT  OF   PLANTS 


331 


ous  rhizoids  (Fig.  228).  This  gametophyte  often  becomes  heart- 
shaped  (Fig.  228,  C),  owing  to  the  more  rapid  growth  of  the 
cells  that  are  cut  off  from  the  apical  cell.  In  some  genera 
branched  filamentous  or  narrowly  thalloid  growths  are  developed 
that  resemble  the  protonema  of  the  mosses  or  the  thallose  hepat- 


r 


FIG.  228. 


FIG.  229. 


FIG.  228.  Gametophyte  of  the  Filicales:  A,  germination  of  the  spore. 
B,  early  appearance  of  the  thalloid  structure  of  the  gametophyte  owing  to 
the  formation  of  an  apical  cell,  x.  C,  mature  gametophyte — an,  antheridia; 
ar,  archegonia;  r,  rhizoids;  v,  apical  cell  or  growing  point. 

FIG.  229.  Structure  in  reproductive  organs:  A,  antheridium  as  seen  in 
section,  just  before  the  discharge  of  the  gametes.  B,  male  gamete.  C, 
section  view  of  archegonium — g,  female  gamete. 

ics.  The  gametophyte  usually  lives  but  a  few  months,  although 
in  some  species  they  may  endure  for  years,  multiplying  exten- 
sively by  gemmae,  and  so  form  conspicuous  mats  upon  the  moist 
trunks  and  rocks.  The  archegonia  and  antheridia  are  usually 
borne  upon  the  same  gametophyte.  Some  genera,  however,  are 


332  THE  YOUNG   SPOROPHYTE 

strictly  dioecious,  producing  small  antheridial  or  male  gameto- 
phytes  and  larger  archegonial  or  female  gametophytes.  Small 
male  gametophytes  occur  not  uncommonly  among  any  of  the 
genera,  owing  doubtless  to  their  poorer  nourishment.  The 
sexual  organs  are  developed  upon  the  under  side  of  the  gameto- 
phyte  (Fig.  228,  C),  probably  because  this  position  is  of  advantage 
in  keeping  them  in  contact  with  any  water  that  may  fall  upon  the 
earth  and  thus  preventing  their  drying  out.  The  antheridia 
develop  before  the  archegonia,  often  appearing  while  the  game  to  • 
phyte  is  quite  small.  They  usually  project  from  the  surface  of 
the  thallus  as  spherical  bodies  covered  with  a  single  layer  of 
chlorophyll-bearing  cells  which  inclose  usually  from  thirty-two 
to  sixty-four  gamete-bearing  cells  (Fig.  229,  A).  The  male 
gametes  are  discharged  as  in  the  Bryophyta — the  swelling  of  the 
antheridium  causes  the  rupture  or  throwing  off  of  the  apical  cell 
and  the  extrusion  of  the  inner  cells  as  a  mucilaginous  mass.  The 
gametes  are  large  spirally-coiled  bodies  provided  with  numerous 
cilia  and  the  larger  posterior  coils  enclose  a  delicate  sac  containing 
the  remains  of  the  nourishment  stored  in  the  mother  cell  (Fig. 
229,  B).  The  archegonia  are  developed  on  the  older  prothallia, 
usually  just  back  of  the  growing  point  (Fig.  228,  C).  The  neck 
of  the  archegonia,  consisting. of  four  rows  of  cells,  projects  from 
the  under  surface  of  the  prothallium  and  is  usually  curved  back 
from  the  growing  point,  while  the  basal  portion  containing  the 
female  gamete  remains  buried  in  the  tissues  of  the  prothallium 
(Fig.  229,  C).  When  the  female  gamete  is  mature  the  canal 
cells  become  mucilaginous  and  the  lip  cells  open  as  in  the  arche- 
gonium  in  the  mosses. 

(c)  Germination  of  the  Gametospore. — The  gametospore  ger- 
minates in  a  week  or  more  after  fertilization,  usually  dividing  very 
regularly  first  by  a  vertical  and  later  by  a  transverse  division  into 
four  cells.  Each  of  these  cells  forms  a  primary  organ  of  the  young 
sporophyte.  Of  the  two  outer  cells,  the  one  near  to  the  grow- 
ing point  of  the  prothallium  (Fig.  230,  A,  c)  by  repeated  divisions 
produces  the  first  leaf  or  cotyledon,  while  the  other  one  forms 
the  first  root  (Fig.  230,  A,  r).  Of  the  two  lower  cells,  the  one 
directly  below  the  leaf  cell  gives  ryse  to  the  stem  (Fig.  230,  A ,  s) 


DEVELOPMENT   OF   PLANTS 


333 


and  the  other  one  to  the  foot  (Fig.  230,  A,  f).  The  growth 
of  these  four  cells  results  at  first  in  the  formation  of  a  rather 
globular  sporophyte,  but  soon  the  growing  point  of  the  young 


FIG.  230.  Germination  of  the  gametospore :  A,  section  of  archegonium, 
after  fertilization,  showing  the  four-celled  stage  of  the  germinating  gameto- 
spore. The  cell  r  by  repeated  division  forms  the  first  root,  c  forms  the  first 
leaf,  s  forms  the  stem  and  /  the  foot;  x,  apical  cell  of  prothallium.  B,  later 
stage.  The  young  sporophyte  rupturing  the  archegonium  or  calyptra, 
lettering  as  in  A. 

root  grows  through  the  calyptra, 'turns  downward  and  pene- 
trates the  soil  (Fig.  230,  B).     This  is  followed  by  emergence 


FIG.  231.  Older  sporophyte  that  has  developed  two  roots,  r,  and  is  un- 
folding the  second  leaf,  /,  but  still  attached  to  the  withering  gametophyte 
or  prothallium,  p;  rh,  rhizoids. 


334  EQUISETALES   OR   HORSETAILS 

of  the  cotyledon,  which,  however,  curves  upward  and  spreads 
out  its  blade  to  the  light  (Fig.  231).  Note  in  Fig.  230  that  the 
necessities  of  the  plant  have  determined  the  position  of  these 
four  organs.  The  foot  is  formed  in  contact  with  the  bulk  of  the 
food  in  the  pro  thallium,  the  root  next  the  ground  which  it  reaches 
at  once,  the  leaf  easily  reaches  the  light  since  the  root  has  already 
ruptured  the  calyptra,  and  since  the  leaf  is  near  the  margin  of 
the  prothallium,  and  finally  the  stem  is  in  a  position  of  advantage 
(Fig.  230,  B)  for  growing  straight  up  or  horizontally.  The  young 
sporophyte,  which  has  up  to  this  time  drawn  its  nourishment  by 
means  of  the  foot  from  the  gametophyte,  is  now  in  a  position  to 
care  for  itself.  The  prothallium  soon  withers,  leaving  the  sporo- 
phyte an  independent  and  self-supporting  plant.  The  first  leaf  is 
small  and  usually  bears  little  resemblance  to  those  of  the  mature 
plant,  but  as  the  stem  elongates,  new  leaves  are  formed  which 
gradually  become  larger  and  each  succeeding  one  resembles  more 
closely  the  adult  form.  The  foot  disappears  with  the  withering 
of  the  gametophyte,  and  this  is  soon  the  fate  of  the  primary  root, 
but  numerous  secondary  roots  are  formed  along  the  stems  as  it 
continues  to  creep  along  on  or  near  the  surface  of  the  soil.  This 
growth  goes  on  from  two  to  several  years  before  the  plant  is 
prepared  to  develop  sporangia  upon  the  leaves,  page  282. 

Order  3.  Equisetales.  The  Horsetails 
114.  General  Characters. — This  small  group  of  Pteridophyta, 
comprising  but  a  single  genus  of  about  twenty-five  species,  is  but 
a  remnant  of  an  extensive  group  of  plants  that  flourished  in  the 
coal  period  and  that  formed  conspicuous  features  of  the  vegeta- 
tion at  that  time.  The  structure  of  these  earlier  forms  has  been 
so  perfectly  preserved  in  fossil  remains  that  they  give  a  better 
idea  of  the  relationship  of  the  group  than  could  be  obtained  from 
the  living  plants.  Forms  allied  to  Equisetum  doubtless  formed 
during  the  coal  period  of  the  earth  large  forests  and  attained  a 
height  of  sixty  to  ninety  feet  and  perhaps  three  feet  in  diameter. 
The  species  that  survive  to-day  are  rush-like  plants  that  rarely 
exceed  a  foot  in  height,  though  a  single  tropical  form  supports 
its  delicate  stem  upon  other  vegetation  and  so  attains  a  length 


DEVELOPMENT   OF    PLANTS 


335 


of  over  thirty-five  feet.  Species  of  Equisetum  are  of  common 
occurrence  in  nearly  all  countries,  living  in  shallow  ponds, 
swamps,  and  marshes  or  drier  soils. 

At  first  sight  they  show  little  suggestion  of  fern  relationship 
(Fig.  232)  and  impress  one  as  being  singularly  out  of  harmony 


FIG.  232.  A  common  horsetail,  Equisetum  arvense:  a,  the  green  branch- 
ing plant  that  lives  through  the  summer — I,  scale  leaves;  r,  rhizome  or  under- 
ground stem  with  tuberous  storage  organs,  b,  early  spring  shoot  that  bears 
a  spike  or  strobilus  of  modified  spore-bearing  leaves,  sp.  This  stem  is  of  a 
light  brown  color  and  withers  after  the  spores  are  shed. — H.  O.  Hanson. 

with  our  common  plants,  as  though  they  were  indeed  relics  of 
a  past  age.  The  large  leaves,  which  were  so  characteristic  of 
the  Filicales,  are  reduced  to  minute  papery  scales  in  this  order 


336  STRUCTURE   OF  EQUISETUM 

and  take  little  or  no  part  in  photosynthesis.  These  scale  leaves 
are  arranged  with  great  regularity  at  the  nodes  of  the  stem,  and 
owing  to  their  close  association  they  grow  together,  forming  a 
papery  sheath  with  teeth-like  points  about  the  stem  (Fig.  232,  Q. 
It  is  noteworthy  that  in  allied  fossil  forms  large  chlorophyll- 
bearing  leaves  occurred. 

The  stems  are  of  two  kinds,  subterranean  rhizomes  that  branch 


FIG.  233.  Cross-section  of  a  portion  of  the  stem  showing  its  grooved 
character  and  stereome  confined  to  the  ridges:  a,  air  spaces;  e,  endodermis, 
inside  of  which  are  shown  three  bundles. 

extensively  through  the  soil,  and  aerial  stems  that  arise  as 
branches  from  the  rhizomes.  The  aerial  stem  is  simple  or 
branched  and  is  characterized  by  nodes  made  conspicuous  by  the 
sheathing  teeth-like  leaves  and  strongly  furrowed  internodes.  In 
such  species  as  branch,  these  organs  originate  with  great  regular- 
ity in  the  axils  of  the  leaves  and  perforating  the  sheathing  leaves 
produce  a  bushy  symmetry  that  caused  the  name  of  Equisetum 
or  horsetail  to  be  applied  to  these  plants.  The  epidermal  cells 
are  hard  and  rough,  owing  to  the  abundant  deposit  of  silica  in 
the  cell  walls.  For  this  reason  certain  species  were  used  in  early 
times  for  scouring  purposes  and  so  they  became  popularly  known 
as  scouring  rushes.  Complete  silicious  casts  of  the  epidermal 


DEVELOPMENT   OF   PLANTS  337 

walls  and  stomata  can  be  obtained  by  treating  the  tissue  as  noted 
in  the  diatoms.  The  arrangement  of  the  leaves  and  branches  and 
the  distribution  of  the  vascular  bundles  and  other  tissues  presents 
a  mathematical  regularity  unexcelled  in  any  plant.  As  in  the 
Bryophyta,  the  elongation  of  the  stem  is  effected  by  a  single 
apical  cell  which  cuts  off  with  extreme  regularity  the  cells  from 
which  the  various  tissues  and  organs  of  the  plant  are  formed 
The  very  rudimentary  vascular  bundles  are  arranged  around  a 
large  pith  (Fig.  233).  They  rarely  show  secondary  growth 
through  the  activities  of  a  cambium,  although  this  was  a  marked 
feature  of  many  of  the  extinct  species.  These  bundles  lie  directly 
below  each  ridge  of  the  stem  and  run  downward  through  the 
internode  to  the  next  lower  node,  where  they  divide  into  two  equal 
parts.  One  of  these  branches  joins  a  similar  branch  from  the 
adjacent  bundle  and  continues  straight  down  to  the  next  node, 
where  the  branching  is  repeated.  The  rudimentary  bundles  of 
the  leaves  also  join  on  to  the  bundles  of  the  stem  at  the  node. 
Owing  to  this  regularity  of  branching,  the  leaves  and  vascular 
bundles  alternate  in  each  succeeding  node  and  the  vascular  system 
assumes  a  cylindrical  form  composed  of  oblong  six-sided  figures. 
Thick-walled  stereome  cells  are  developed  in  the  ridges  and  con- 
stitute the  principal  mechanical  tissue  of  the  stem,  while  the 
stomata  and  the  chlorophyll  apparatus  are  largely  confined  to 
the  grooves.  The  stems  are  very  light,  owing  to  absorption  of 
the  larger  part  of  the  pith  and  certain  regions  of  the  cortex  (Fig. 

233,  a). 

(a)  The  Spore-bearing  Leaves  or  Sporophylls. — The  sporangia 
are  formed  only  upon  special  organs,  probably  leaves  or  sporo- 
phylls,  that  are  arranged  in  a  compact  cone  or  strobilus  (plu. 
strobili)  at  the  end  of  the  stem  (Fig.  232,  sp).  In  some  species  the 
strobilus  is  borne  upon  a  special  branch  that  does  not  contain 
chlorophyll,  though  variously  colored,  and  that  withers  away  as 
soon  as  the  spores  are  shed  (Fig.  232,  b).  In  other  cases  the  stro- 
bilus appears  at  the  tip  of  the  ordinary  green  shoot.  This  vari- 
ation is  doubtless  associated  with  the  season  of  the  year  at  which 
the  strobili  appear.  The  species  characterized  by  the  special 
branches  produce  these  early  in  the  spring  when  the  temperatures 


338  SPOROPHYLLS   OF   EQUISETUM 

are  not  favorable  for  the  work  of  the  green  shoots.  Note  in  this 
connection  the  significance  of  the  coloration.  Species  with  the 
strobili  upon  the  ordinary  plants  form  these  structures  later  in 
the  season  when  the  temperatures  are  favorable  for  photosyn- 
thesis. The  development  of  the  sporangia  upon  specialized 
sporophylls  that  are  grouped  together  at  definite  points  on  the 
plant  is  one  of  the  significant  departures  that  appears  in  the  evo- 
lution of  the  plant.  This  distribution  of  labor  that  is  seen  in  the 
setting  aside  of  certain  groups  of  leaves  for  spore  production 
will  appear  in  all  the  succeeding  groups ;  and  the  arrangement  and 
structure  of  the  sporophylls  will  steadily  become  more  and  more 
complex  until  a  point  is  reached  where  they  are  popularly  called 
a  flower,  although  this  term  can  be  just  as  correctly  applied  to 
the  strobilus  appearing  in  Equisetum  and  succeeding  groups. 
The  sporophylls  of  Equisetum  originate  at  the  nodes,  as  in  the 
case  of  the  scale  leaves,  but  as  they  enlarge  the  apical  portion 
spreads  out  like  a  shield.  The  internodes  do  not  elongate  and 
separate  the  sporophylls  to  any  considerable  extent,  consequently 
the  shields  become  six-sided  through  contact  with  the  adjacent 
sporophylls. 

(b)  Character  of  Sporangia  and  Spores. — The  sporangia  appear 
as  rather  elongated  sacs  on  the  under  side  of  the  shields   (Fig. 
234,  2)   and  at  maturity  open  by  a  longitudinal  cleft.     The 
structure  of  the  spores  is  of  especial  interest.     The  outer  wall 
of  these  spores  is  thickened  in  spiral  bands,  and  owing  to  the 
dissolution  of  the  thin  portion  of  the  wall  separating  these  bands, 
the  outer  coat  uncoils  and  appears  as  four  bands,  with  spoon-like 
ends,  attached  to  the  spore  at  one  point  (Fig.  234,  3).     These 
bands  or  elaters  are  very  hygroscopic  and  their  movements  assist 
in  rupturing  the  sporangium,  but  their  special  significance  is  seen 
in  the  fact  that  the  elaters  become  entangled  and  so  several  spores 
are  carried  away  together  by  the  wind.     The  meaning  of  this 
arrangement  will  appear  directly. 

(c)  Germination  of  the  Spore. — The  gametophyte  produced  from 
these   spores   (Fig.  235)   is   an  irregularly  lobed    thallus  more 
suggestive  of  the  irregular  thallus  of  an  hepatic  or  the  leaves 
of  a  moss  plant,  or  the  lobed  gametophyte  of  certain  species  of 


DEVELOPMENT   OF   PLANTS 


339 


the  Ophioglossales  than  of  the  prothallium  of  the  Filicales.  The 
most  important  feature  about  the  gametophytes  is  the  fact  that 
they  are  as  a  rule  dioecious  and  of  two  sizes,  the  smaller  ones 
bearing  only  antheridia  and  the  larger  only  archegonia.  This 
difference  in  the  nature  of  the  gametophytes  is  largely  due  to 
nutrition  as  noted  among  the  Filicales,  the  well-nourished  ones 
being  the  female.  In  fact  antheridia  may  appear  upon  the  female 
gametophyte  as  a  result  of  insufficient  nourishment  during  its 


FIG.  234.  FIG.  235. 

FIG.  234.  Sporophylls  and  spores  of  Eguisetum:  2,  sporophylls  viewed 
from  outer  and  inner  side,  showing  form  and  attachment  of  sporangia  and 
the  central  stalk  attaching  the  sporophyll  to  the  strobilus.  3,  spores  with 
elaters  expanded  in  a  and  partially  coiled  in  b. 

FIG.  235.  Female  gametophyte  of  Equisetum  bearing  several  archegonia 
and  leaf-like  lobes.  At  right  male  gamete. — After  Sadebeck. 

later  development.  Thus  we  see  that  the  germination  of  these 
spores,  which  are  apparently  exactly  alike,  is  controlled  by  a  defi- 
nite stimulus,  just  as  was  the  case  in  the  formation  of  zoospores 
and  gametes  among  the  lower  green  algae  (page  185).  It  is 
noteworthy  that  some  of  the  extinct  species  of  this  group  actually 
stored  more  food  in  certain  spores  than  in  others,  so  that  they 
came  to  differ  somewhat  in  size.  This  habit  is  well  established 
in  some  of  the  living  Filicales  and  will  also  be  noted  in  the  next 
order.  As  a  consequence  of  this  tendency,  the  nature  of  the 
gametophyte  developed  from  the  spore  is  no  longer  a  matter  of 
chance.  The  larger  spores,  called  megaspores,  by  reason  of  the 
more  abundant  food  produce  female  gametophytes,  while  the 
smaller  spores,  microspores,  form  small  gametophytes  bearing 
only  antheridia.  The  significance  of  this  tendency  to  produce 


340  REPRODUCTION  OF  EQUISETUM 

megaspores  and  microspores  will  be  seen  in  the  disussion  of  the 
fourth  order,  but  attention  is  called  to  it  here  because  the  Equise- 
tales  show  very  clearly  how  such  a  condition  came  about.  It 
would  appear  probable  that  the  spores  of  Equisetum,  although 
exactly  alike  as  far  as  we  can  see,  must  have  already  undergone 
some  physiological  change  which  predisposes  them  to  develop 
either  male  or  female  gametophytes.  If  this  is  not  the  case  it 
would  be  difficult  to  explain  the  common  occurrence  of  dioecious 
gametophytes  in  Equisetum  and  the  rarity  of  such  an  occurrence 
among  the  Filicales.  It  would  appear  reasonable  to  suppose  that 
the  living  substance  in  the  spore  of  the  Filicales  is  not  so  highly 
organized  and  therefore  not  so  readily  influenced  by  external  con- 
ditions, while  the  composition  of  the  spore  of  Equisetum  is  of 
such  a  nature  that  the  amount  of  food  placed  at  its  disposal  pro- 
foundly affects  its  germination  and  development.  Perhaps  this 
physiological  differentiation  of  the  spores  led  to  their  appro- 
priating different  amounts  of  food  during  their  formation  in  the 
sporangium,  and  so  they  finally  came  to  be  distinguished  as 
large  and  small  spores,  as  noted  above.  The  antheridia  and 
archegonia  present  essentially  the  same  features  as  were  noted 
in  the  Filicales,  and  fertilization  is  effected  in  the  same  manner. 
(d)  The  Germination  of  the  Gametospore. — The  most  note- 
worthy departure  in  the  germination  of  the  gametospore  is  seen 
in  the  limited  growth  of  the  stem.  The  early  stages  are  similar 
to  those  of  the  Filicales,  but  after  the  stem  has  formed  a  few 
nodes  with  three  leaves  each,  it  is  replaced  by  a  stem  that  devel- 
ops at  its  base.  This  second  stem  attains  a  somewhat  larger  size, 
but  is  finally  replaced  in  the  same  manner  by  a  third  shoot.  In 
this  way  several  stems  are  formed  until  finally  one  is  developed 
that  penetrates  the  ground  and  forms  the  characteristic  rhizome 
of  the  mature  sporophyte.  These  plants  with  their  scale  leaves 
reduced  to  protective  organs,  sunken  stomata  and  chlorenchyma 
confined  to  the  grooves  of  the  inter  nodes,  present  an  extreme 
form  of  xerophytic  structure  and  form  a  sharp  contrast  with  the 
large  and  usually  thin-leaved  Filicales.  While  they  occur  in  dry 
localities,  to  which  conditions  their  structures  admirably  adapt 
them,  they  are  of  more  common  occurrence  in  moist  and  wet 


DEVELOPMENT   OF   PLANTS  341 

places.  This  peculiar  distribution  of  the  species  of  Equisetum 
has  not  been  explained.  It  is  evident  that  these  plants,  like  the 
rushes  and  sedges  of  our  marshes  and  shallow  ponds,  are  often 
exposed  to  intense  heat  and  light,  which  would  cause  an  excessive 
transpiration.  Possibly  these  plants  are  not  able  to  absorb 
water  rapidly  owing  to  the  limited  amount  of  conducting  tissues 
and  to  the  exclusion  of  the  atmosphere  by  the  water  which  sur- 
rounds the  roots  (see  pages  45,  54),  consequently  they  are  at 
the  same  disadvantage  and  require  the  same  protective  devices 
as  plants  living  in  arid  localities. 

Order  4.     Lycopodiales.    The  Club  Mosses 
115.  General  Characters. — The  members  of  this  group  are 
popularly  known  as  the  club  mosses  owing  to  their  small  moss- 
like  leaves  and  the  arrangement  of  the  spore-bearing  leaves  into 


FIG.  236.  A  common  club  moss,  Lycopodium  annotinum,  with  creeping 
stem  and  erect  branches  covered  with  small  moss-like  leaves:  s,  strobilus, 
r,  roots. — H.  O.  Hanson. 


342  THE   LYCOPODIALES 

club-like  strobili  or  cones  (Fig.  236).  The  gametophyte  or  sexual 
generation  is  also  suggestive  of  the  mosses.  Particularly  is  this 
true  of  the  male  gamete,  which  is  small  and  not  spirally  coiled 
and  it  is  apparently  biciliate  (Fig.  240).  » 

The  Lycopodiales  are  largely  tropical,  though  a  small  number 
of  forms  are  of  common  occurrence  in  temperate  regions.  The 
stems  are  erect  or  creeping  and  rather  small,  but  owing  to  their 
prolonged  growth  and  extensive  branching  they  often  form  con- 


FIG.  237.     Cross-section  of  a  portion  of  the  stem  of  Lycopodium,  showing 
the  centrally  arranged  vascular  bundles. 

spicuous  and  attractive  colonies.  The  tissues  of  the  stem  do 
not  materially  differ  from  those  of  the  common  fern,  though  the 
vascular  bundles  from  the  leaves  generally  unite  in  the  stem  to 
form  a  central  core  of  xylem  surrounded  by  phloem.  This 
solid  core  of  xylem  may  be  broken  up  in  various  ways  by  the 
development  in  it  of  masses  of  pith-like  cells.  Sometimes  these 
pith  cells  form  plates  alternating  with  the  xylem  (Fig.  237). 
The  work  performed  by  the  leaves,  as  in  the  Equisetales,  is 
usually  of  two  kinds,  namely,  photosynthesis  by  the  green  foliage 
leaves  and  spore  production  by  the  leaves  that  usually  form 


DEVELOPMENT   OF   PLANTS 


343 


strobili  at  the  tips  of  certain  branches.     But  a  single  sporangium 
is  associated  with  each  of  these  sporophylls  (Fig.  239). 

The  club  mosses  are  a  very  much  larger  group  than  the  Equise- 
tales,  but  like  the*  latter  group,  they  are  a  remnant  of  a  highly 
developed  and  widely  distributed  race.  Fossil  remains  indicate 
that  the  ancient  allies  of  these  plants  were  conspicuous  features 
of  an  earlier  vegetation,  with  palm-like  trunks  100  feet  in  height 
and  three  feet  in  diameter,  and  bearing  a  crown  of  long  narrow 
leaves  that  attained  a  length  of  three  feet.  They  reached  their 


FIG.  238.  FIG.  239. 

FIG.  238.     Phylloglossum  Drummondi. — After  Pritzel. 

FIG.  239.  Strobilus  and  sporophylls  of  Lycopodium:  2,  strobilus.  3,  a 
leaf  or  sporophyll  from  the  strobilus  enlarged  and  showing  attached  spo- 
rangium. 4,  a  spore  greatly  magnified. 

greatest  abundance  in  the  coal  age  and  thence  gradually  declined, 
being  crowded  out  by  the  more  specialized  seed  plants.  There 
are  two  important  families  of  the  Lycopodiales :  I ,  Lycopodiaceae ; 
2,  Selaginellaceae. 

116.  Family  i.  Lycopodiaceae. — With  but  one  exception  the 
members  of  this  family  belong  to  the  genus  Lycopodium,  com- 
monly known  as  the  club  moss,  ground  or  running  pine,  ground 


344  SIGNIFICANCE   OF   PHYLLOGLOSSUM 

fir  or  hemlock  (Fig.  236).  These  plants  are  very  well  repre- 
sented in  our  open  woods,  where  they  often  form  conspicuous 
colonies  owing  to  the  extensive  branching  and  prolonged  growth 
of  the  stems  which  creep  over  or  through  the  ground,  sending 
up  numerous,  erect  branches  and  giving  off  roots  that  branch 
with  great  regularity.  Various  species  of  Lycopodium  are  ex- 
tensively gathered  for  decorations  since  the  aerial  portions  of 
the  stems  are  thickly  clothed  with  small  moss-like  leaves.  The 
second  genus  of  the  family  (Phylloglossum)  contains  but  one 
species,  which  is  found  in  New  Zealand  and  Australia.  It  is  of 
interest  because  it  is  the  most  simple  fern  known  (Fig.  238), 
consisting  of  a-  few  narrow  leaves,  rudimentary  strobilus  and 
poorly  developed  tissues.  The  simple  strobilus  of  this  plant  is 
suggestive  of  the  sporophyte  of  Anthoceros  and  it  also  recalls  the 
spore-bearing  spike  of  Ophioglossum.  If  the  sterile  cells  separat- 
ing the  spore-forming  cells  in  Anthoceros  had  increased  in  number 
so  as  to  project  somewhat  and  thus  form  rudimentary  leaf -like 
outgrowths,  then  it  would  have  resembled  the  strobilus  of  Phyl- 
loglossum. To  be  sure  the  sporangia  of  the  latter  plant  are  borne 
upon  the  leaves  but  in  the  next  family  they  arise  from  the  stem 
as  in  Anthoceros.  Ophioglossum  shows  the  increase  of  the  sterile 
cells  between  the  spore-bearing  cells  but  this  did  not  result  in 
leaf  development;  so  it  occupies  an  intermediate  position  in  this 
trend  of  development.  It  is  certainly  a  striking  fact  that  these 
three  groups,  though  widely  separated,  exhibit  the  same  type  of 
development. 

The  sporangia  of  the  Lycopodiaceae  are  large  considering  the 
size  of  the  leaf,  and  usually  appear  on  specialized  leaves  that  are 
arranged  in  strobili  on  the  ends  of  erect  branches  (Fig.  239). 
The  sporangia  open  by  a  longitudinal  cleft  and  the  spores  are 
produced  in  such  quantities  in  some  of  the  forms  that  they  are 
of  commercial  importance,  being  used  for  flashlight  effects  and 
sold  as  lycopodium  powder. 

(a)  The  Gametophyte  of  the  Lycopodiaceae. — The  most  inter- 
esting feature  about  the  club  mosses  is  the  sexual  generation, 
because  it  indicates  that  these  forms  may  have  been  derived  from 
the  same  ancestry  as  the  mosses  or  at  least  from  a  very  primitive 


DEVELOPMENT   OF   PLANTS  345 

stock.  In  certain  species  the  spores  produce  a  rather  cylindrical 
erect  gametophyte  which  terminates  in  a  number  of  radially- 
arranged  green  leaf -like  lobes,  among  which  the  archegonia  and 
antheridia  are  produced  (Fig.  240).  This  structure  has  been 
compared  to  a  miniature  leafy  moss  plant.  This  suggestion  of 
relationship  is  strengthened  by  the  resemblance  of  the  male 
gametes  to  those  of  the  mosses,  the  sperms,  so  far  as  they  have 
been  observed,  being  small,  almost  straight  bodies,  and  provided 
with  two  cilia  (Fig.  240).  The  archegonia  may  also  be  more 
primitive  than  those  of  the  ferns  in  possessing  several  neck  canal 


FIG.  240.  Gametophyte  of  Lycopodium  as  seen  in  section:  an,  antheridia; 
ar,  archegonia.  The  young  sporophyte  or  embryo  developed  in  one  of  the 
archegonia  consists  of  a  foot,  /;  a  root,  r;  and  a  stem,  st,  bearing  a  leaf  on 
either  side;  s,  the  suspensor,  which  has  pushed  the  sporophyte  early  in  its 
development  into  the  tissues  of  the  gametophyte.  Above  a  single  male 
gamete  is  figured. — After  Bruchmann. 

cells  and  five  instead  of  four  rows  of  neck  cells.  The  gameto- 
phyte appears  in  all  species  examined  to  be  associated  with 
symbiotic  fungi,  as  in  Ophioglossales.  The  spores  will  not  de- 
velop unless  they  come  in  contact  with  a  suitable  fungus  and  this 
has  led  in  many  forms  to  a  subterranean  and  chlorophylless 
development  of  the  gametophyte. 

(b)   The  Germination  of  the  Gametospore. — The  germination  of 

the  gametospore  is  characterized  by  new  departures  that  will 

continue  on  through  the  seed  plants.     The  gametospore  enlarges 

and  divides  into  two  cells  by  a  transverse  wall.     The  outer  of 

23 


346  EMBRYO   OF   LYCOPODIUM 

the  two  daughter  cells  takes  no  part  in  the  formation  of  the 
sporophyte,  though  elongating  and  often  dividing  several  times- 
These  cells  are  termed  the  suspensor  (Fig.  240,  s)  and  function 
in  pushing  the  lower  daughter  cell  down  into  the  nourishing  tissue 
of  the  gametophyte.  The  lower  daughter  cell,  by  a  series  of 
divisions  that  at  first  resemble  the  common  ferns,  forms  the 
young  sporophyte.  This  consists  of  a  stem  with  one  or  two 
cotyledons,  a  massive  foot  and  finally,  at  a  late  period  in  the 
development  in  the  sporophyte,  of  a  root  (Fig.  240).  The  tardy 
development  of  the  root  has  been  cited  as  an  indication  of  the 
origin  of  the  lycopods  from  very  primitive  ancestors,  in  which 
the  formation  of  the  root  had  not  become  established.  It  may 
also  be  due  to  the  abundant  food  stored  in  the  gametophyte  and 
hence  the  development  of  the  root  might  well  be  delayed  until 
this  store  is  in  part  exhausted.  The  growth  of  the  sporophyte  is 
very  slow  and  it  remains  as  a  parasite  upon  the  gametophyte  for 
a  long  time,  even  for  years  in  some  of  the  subterranean  forms 
(see  Ophioglossales) .  The  elongation  of  the  stem  and  root  is 
effected  by  the  division  of  several  cells  rather  than  by  one  apical 
cell,  as  in  previous  cases,  a  feature  to  be  noted  in  the  seed  plants. 
117.  Family  2.  Selaginellaceae. — This  family  includes  but  a 
single  genus,  Selaginella,  of  over  600  species.  Only  a  few  forms 
occur  in  the  temperate  regions,  the  majority  being  confined  to 
tropical  countries,  where  they  often  form  one  of  the  most  attrac- 
tive features  of  the  forest  vegetation  owing  to  the  symmetry  of 
their  branching  and  the  rare  delicacy  of  their  foliage  (Fig.  241). 
For  these  reasons  they  are  extensively  cultivated  and  familiar 
objects  in  conservatories  and  florists'  shops.  The  so-called  res* 
urrection  plant,  Selaginella  lepidophylla,  lives  in  the  very  arid 
sections  of  the  southwestern  United  States,  and  during  drought 
reduces  its  surface  to  a  nest-like  ball  by  rolling  up  its  branches 
into  tight  coils.  In  this  condition  it  appears  as  a  brownish  dead 
mass.  When  moistened,  the  absorption  of  water  causes  the 
branches  to  quickly  uncoil  and  also  renders  the  tissues  trans- 
lucent, so  that  the  green  color  of  the  chloroplasts  can  be  seen. 
These  reactions  occur  even  in  the  dead  plants  and  so  create  the 
impression  that  they  have  returned  to  life.  Many  of  the  species 


.DEVELOPMENT   OF   PLANTS  347 

of  Selaginella  have  a  creeping  habit  like  that  of  Lycopodium, 
and  the  aerial  stems  are  covered  with  leaves  arranged  in  several 
rows,  very  frequently  in  four  rows  of  two  large  and  two  smaller 
leaves  (Fig.  241,  lA).  Usually  but  a  single  chloroplast  appears 
in  each  cell,  as  already  noticed  in  several  algae  and  in  the  Antho- 


FIG.  241.  A  common  cultivated  Selaginella:  it  habit  of  the  plant — sf 
strobili;  &,  a  branch  bearing  roots,  r.  lA,  portion  of  stem,  showing  leaf  ar- 
rangement.— H.  O.  Hanson. 

cerotales.  The  roots  are  frequently  developed  from  the  end  of 
naked  branches  that  extend  from  the  stems  to  the  ground  (Fig. 
241,  r). 

(a)  Sporangia  and  Spores. — The  sporangia  are  borne  on  the 
stems  in  the  axils  of  the  leaves  which  form  a  strobilus,  as  in 
Lycopodium.  However,  they  differ  radically  from  those  of  the 
Lycopodium,  in  that  two  kinds  of  spores  are  formed,  small  spores 
or  microspores  and  larger  ones  or  megaspores.  The  formation 
of  two  kinds  of  spores,  or  heterospory,  is  not  confined  to  the 
Lycopodiales.  It  occurs  among  certain  genera  of  the  Filicales 


348 


SPOROPHYLLS   OF  SELAGINELLA 


and,  as  has  been  stated,  characterizes  some  of  the  fossil  Equise- 
tales.  The  megaspores  are  generally  formed  at  the  base  of  the 
strobilus  and  the  microspores  occupy  the  upper  sporangia  (Fig. 
242).  The  sporangia  are  called  microsporangia  and  megaspor- 
angia  accordingly  as  they  contain  small  or  large  spores  and  for 
the  same  reason  the  leaves  may  be  designated  as  micro-  and 


FIG.  242.  Sporophylls  and  spores  of  Selaginella:  2,  strobilus  with  lower 
sporophylls  separated  for  spore  dissemination.  3,  megasporophyll  with 
sporangium  containing  four  megaspores.  4,  microsporophyll.  5,  mega- 
spore  enlarged.  6,  microspore  equally  magnified.  6-4,  more  enlarged  view 
of  the  microspore.  The  triangular  surfaces  of  the  spores  show  that  these 
spores  have  been  formed  in  tetrads  from  a  spore  mother  cell  as  in  previous 
groups. 

mega-sporophylls.  The  two  kinds  of  spores  originate  in  the 
same  manner  as  previously  noted  and  the  difference  in  size  is 
due  to  the  amount  of  food  which  they  receive.  In  the  case  of 
the  microspores,  the  numerous  mother  cells  of  the  sporangia  form 
four  spores  each  in  the  usual  manner:  but  in  the  megasporangia 
only  one  of  the  mother  cells  divides  in  this  manner,  and  in  some 
cases  only  one  spore  is  formed.  The  other  mother  cells  do  not 
develop  and  are  ultimately  consumed  in  nourishing  the  mega- 
spores. As  a  result  of  the  large  amount  of  food  transferred  to 
the  megasporangium,  both  it  and  especially  the  one  to  four 
spores  become  much  larger  than  the  others  (Fig.  242,  3-6). 
The  physiological  differentiation  of  the  spores  noted  in  Equisetum 


DEVELOPMENT   OF   PLANTS  349 

has  here  gone  a  step  further.     They  are  so  constituted  that  they 
now  appropriate  all  the  food  in  their  respective  sporangia. 

(b)  The  Germination  of  the  Spores. — The  most  important  and 
suggestive  feature  in  the  life  history  of  Selaginella  appears  in 
the  germination  of  the  spores.  Th^^s^res  germinate  in  the 
sporangia  and  not  after  being  shed,  as  in  the  other  orders.  This 
growth  is  somewhat  different  from  previous  cases  in  that  at  first 
there  is  no  outgrowth  of  the  spore  but  simply  the  formation  of  a 
varying  number  of  nuclei  within  the  spore.  Later  these  nuclei 
develop  cell  walls  and  subsequent  growth  ruptures  the  spore. 
In  this  way  the  microspore  forms  two  cells  while  in  the  sporangia, 
a  small  one  often  compared  to  a  remnant  of  the  male  gameto- 
phyte  or  prothallium  and  a  large  cell  which  ultimately  forms  a 
single  antheridium.  At  about  this  stage  of  germination  the 
microsporangium  opens  by  a  vertical  cleft,  permitting  the  scatter- 
ing of  the  spores.  The  larger  cell  of  the  microspores  now  forms 
a  central  mass  of  gamete  mother  cells  which  are  surrounded  by  a 
single  layer  of  wall  cells  (Fig.  243,  i).  The  male  gametes  are  of 
the  same  character  as  noted  in  Lycopodium  and  are  set  free 
by  the  disintegration  of  the  wall  cells.  The  megaspores  begin  to 
germinate  even  before  they  have  reached  their  full  size.  The 
nucleus  of  a  megaspore  divides  repeatedly,  forming  numerous 
nuclei  which  become  arranged  about  the  walls  of  the  young  spore. 
Subsequently  walls  are  formed  about  the  nuclei  at  the  apex  of 
the  spore  (Fig.  243,  2)  and  by  further  division  a  mass  of  cells 
results,  in  the  outer  part  of  which  the  first  divisions  of  the  arche- 
gonia  arise.  At  this  stage  of  development,  or  earlier,  the  mega- 
sporangium  opens  by  a  vertical  cleft  permitting  the  discharge  of 
the  spores.  It  must  be  borne  in  mind  that  this  origin  of  the 
gametophyte  is  of  a  radically  different  nature  from  any  case  pre- 
viously noted.  Heretofore  the  gametophyte  always  had  an  inde- 
pendent existence  and  the  sporophyte  for  varying  periods  of  time 
was  a  parasite  upon  it.  In  Selaginella  you  note  the  beginning 
of  a  reversal  of  this  relation,  for  the  sporangia  remain  green 
and  continue  to  nourish  the  spores  during  their  germination;  in 
other  words,  during  the  early  stages  in  the  development  of  the 
gametophyte.  We  see  that  the  gametophyte  is  becoming  para- 


I 


350 


GAMETOPHYTE  OF  SELAGINELLA 


sitic  upon  the  sporophyte  (see  page  281).  After  the  spores  are 
shed  they  Complete  the  growth  of 'the  gametophyte  by  forming 
the  archegonia  and  filling  the  space  within  the  spores  with  a  soli'd 
mass  of  cells.  This  growth  ruptures  the  spore  walls  at  the  apical 
regions,  thus  exposing  the  archegonia  to  the  male  gametes  (Fig. 
243,  3).  The  archegonia  are  rudimentary  structures  consisting 


FIG.  243.  Gametophyte  and  young  sporophyte  of  Selaginella:  I,  section 
of  a  microspore  that  has  nearly  completed  its  germination — p,  a  single  cell 
that  is  possibly  a  remnant  of  the  large  gametophyte  or  prothallium  of  pre- 
vious groups  of  ferns;  an,  antheridium  consisting  of  a  layer  of  wall  cells 
which  enclose  the  gamete  mother  cells.  lA,  a  male  gamete.  2,  section 
of  a  megaspore,  showing  the  stage  of  germination  that  is  usually  attained 
in  the  sporangium.  3,  mature  female  gametophyte  that  has  ruptured  the 
wall  of  the  megaspore,  thus  exposing  the  archegonia,  ar,  one  of  which  has 
developed  a  young  sporophyte  or  embryo  with  root,  r;  stem  bearing  two 
leaves,  st,  foot,  /,  and  the  suspensor,  s.  4,  young  sporophyte  with  root, 
stem  and  leaves  emerging  from  the  gametophyte. 

of  but  two  neck  cells,  and  a  single  canal  cell  leads  to  the  female 
gamete. 

It  is  evident  that  these  variations  are  of  great  advantage  in 
ensuring  the  perpetuation  of  the  species.  Especially  is  the 
nourishment  of  the  young  gametophyte  by  the  highly  organized 
sporophyte  a  distinct  gain.  In  previous  types  the  formation  of 
the  gametophyte  was  dependent  upon  favorable  conditions,  such 
as  moisture,  light,  temperature,  etc.,  and  it  should  be  said  that 


DEVELOPMENT   OF   PLANTS  351 

only  in  rare  cases  do  the  spores  fall  in  such  favorable  situations 
and  meet  with  such  suitable  conditions  as  to  enable  them  to 
mature  archegonia  and  antheridia,  and  so  provide  for  a  new  sporo- 
phyte.  In  Selaginella  the  formation  of  the  gametophyte  is  en- 
sured by  the  food  supplied  to  it  by  the  sporophyte.  Thus  the 
germination  of  the  spore  is  practically  independent  of  its  sur- 
roundings and  it  can  complete  its  growth  under  a  variety  of  condi- 
tions, even  in  the  dark.  It  should  also  be  noted  that  this  relation- 
ship of  the  two  generations  results  in  a  marked  reduction  in  the  - 
size  of  both  the  male  and  female  gametophyte.  since  they  are  no 
longer  burdened  with  the  work  of  food  construction.  This  re- 
duction will  proceed  very  rapidly  as  we  advance  through  the 
remaining  groups. 

(c)  Germination  of  the  Gametospore. — The  germination*  of  the 
gametospore  is  very  much  like  that  of  Lycopodium.  The  sus- 
pensor  is  much  longer  and  the  young  sporophyte  consists  of 
a  stem  bearing  two  cotyledons,  a  well-developed  foot  and  a  rr  t 
that  is  developed  after  the  other  organs  (Fig.  243,  3).  The  •  jot 
continues  to  absorb  food  from  the  gametophyte  after  the  stem 
and  root  have  emerged,  and  in  this  condition  the  relation  of  the 
sporophyte  to  the  gametophyte  is  strikingly  suggestive  of  a 
sprouting  seed  (Fig.  243,  4).  In  this  connection  it  should  be 
remembered  that  the  young  sporophyte  of  certain  species  remains 
in  the  gametophyte  during  the  winter  or  during  a  drought  in  a 
resting  or  dormant  condition,  thus  resembling  very  closely  the 
seed  structure  to  be  seen  in  the  next  group.  It  may  also  be 
stated  that  fertilization  and  even  the  formation  of  the  young 
sporophyte  may  begin  before  the  spores  are  discharged  from  the 
sporangium,  thus  helping  us  to  understand  how  it  came  about 
that  the  megaspores  were  permanently  retained  in  the  sporan- 
gium as  is  the  case  among  seed  plants.  In  one  of  the  large 
groups  of  Pteridophyta  that  have  become  extinct,  it  is  note- 
worthy that  both  the  gametophyte  and  young  sporophyte  were 
retained  in  the  sporangium  so  that  the  seed-like  structures  were 
formed. 


CHAPTER   IX  ' 

DIVISION   IV.     SPERMATOPHYTA  OR  SEED  PLANTS 

118.  General  Characters. — This  is  the  largest  group  of  the 
vegetable  kingdom  and  includes  all  our  common  trees,  shrubs 
and  herbs.  The  relationship  of  these  plants  is  very  imperfectly 
understood  and  the  various  classes,  orders,  etc.,  into  which  they 
are  divided  are  in  part  artificial  and  do  not  therefore  represent 
completely  the  alliances  of  the  groups.  You  have  noticed  that 
the  rporophyte  is  the  most  important  feature  in  the  life  history 
of  the  fern  and  that  the  gametophyte,  subordinate  from  the  start, 
becomes  very  inconspicuous  in  some  of  the  forms.  This  in- 
equality of  the  two  generations  is  more  noticeable  in  the  seed 
plants  where  a  progressive  series  of  variations  result  in  a  more  and 
more  complex  external  and  internal  differentiation  of  the  various 
organs  of  the  sporophyte  and  in  a  steady  reduction  of  the  gameto- 
phyte. Heterospory  that  appeared  in  several  groups  of  the  Pteri- 
dophyta,  becomes  a  constant  characteristic  of  the  seed  plants. 
The  spores  originate  much  as  in  the  preceding  group  but  the 
megaspore  is  nourished  and  permanently  retained  in  the  sporan- 
gium where  it  not  only  forms  the  female  gametophyte  (see  Selagi- 
nella) but  also  develops  the  young  sporophyte.  At  this  point, 
the  most  characteristic  feature  of  the  Spermatophyta  is  seen. 
The  young  sporophyte  or  embryo  usually  ceases  to  grow  while 
still  in  the  gametophyte  and  passes  into  a  resting  or  dormant  con- 
dition and  the  sporangium  which  has  become  variously  modified 
for  the  purpose  of  protecting  the  embryo  is  discharged  from  the 
sporophyte.  This  sporangium  containing  the  embryo  is  called 
the  seed.  Contrast  with  these  features  two  important  differences 
that  appeared  in  Selaginella.  First:  The  spores  of  these  ferns 
are  retained  and  nourished  in  the  sporangia  only  during  the 
early  development  of  the  gametophytes.  Second:  The  young 
sporophyte  of  Selaginella — it  may  also  be  called  the  embryo — 
does  not  normally  pass  into  a  resting  sjate,  but  steadily  grows  on 

352 


DEVELOPMENT  OF  PLANTS  353 

into  the  mature  sporophyte.  The  seed  plants  are  divided  into 
two  subdivisions  based  upon  the  relation  of  the  megasporangia 
to  the  sporophylls:  I.  Gymnospermae,  with  sporangia,  hence 
with  the  seed,  on  the  surface  of  the  sporophyll.  2.  Angiospermae, 
with  sporangia,  hence  with  the  seed,  enclosed  by  the  sporophyll. 
It  should  be  stated  that  this  association  of  these  two  subdivisions 
is  artificial,  the  Gymnosperms  being  more  nearly  related  to  the 
Pteridophyta  than  to  the  Angiosperms. 

Subdivision  i.     Gymnospermae.    Plants  with  Naked  Seeds 

119.  Origin  of  the  Gymnospermae. — This  group  is  principally 
represented  to-day  by  the  cone-bearing  trees.     In  very  ancient 
geological  times,  several  groups  of  primitive  gymnosperms,  now 
extinct,  flourished  and  formed  an  extensive  and  varied  forest 
vegetation.     Fossil  remains  of  these  various  groups  have  been 
wonderfully  well  preserved  and  show  that  they  were  undoubtedly 
related  to  the  Pteridophyta.     The  modern  group  of  gymnosperms 
has  been  derived  from  these  ancient  lines  and  but  a  remnant 
has  survived  the  changes  occurring  upon  the  earth  and  the  com- 
petition with  the  more  highly  specialized  Angiospermae.     A  few 
genera,  however,  are  well  represented  to-day  and  they  are  of 
great  commercial  importance  as  lumber.     The  sporophylls  of 
the  gymnosperms  are  arranged  in  a  strobilus  as  in  the  Equise- 
tales  and  Lycopodiales,  but  the  microspores  and  megaspores  are 
not  associated,  being,  developed  in  separate  strobili.     Attention 
will  be  directed  to  only  two  of  the  more  important  orders:  The 
Cycadales  or  Cycads  and  the  Pinales  or  cone-bearing  trees, 

120.  Order  a.     Cycadales  or  Cycads. — These  plants  are  strictly 
tropical,  though  two  genera  are  subtropical,  Zamla  or  coontie 
in  Florida,  and  Cycas,  often  called  the  sago  palm,  in  China  and 
Japan.     Many  of  them  are  extensively  cultivated  in  green-houses 
owing  to  their  peculiar  growth  and  attractive  foliage  (Fig.  244). 
The  cy cads  include  less  than  100  species  of  what  was  in  geolog- 
ical times,  a  very  extensive  alliance.     They  are  of  special  in- 
terest as  showing  unmistakable  fern  characters  and  because  they 
are  the  most  primitive  of  our  extant  seed  plants.    The  stems 
of  these  plants  are  rather  tuberous,  though  certain  species  are 


354  THE   CYCADS 

i 

decidedly  palm-like  in  appearance,  with  stems  12  to  60  feet  high. 
The  vascular  bundles  are  collateral  and  arranged  around  a  large 
pith  as  noted  in  Botrychium.  A  slight  enlargement  of  the  stem 
is  brought  about  owing  to  the  weak  growth  'of  the  cambium  of 
these  bundles,  but  in  some  genera  the  principal  increase  is  effected 
by  the  formation  of  new  bundles  outside  of  those  first  formed. 
Fern  types  of  bundles  also  occur  in  the  leaves,  cortex  and  strobili 
of  certain  species.  The  foliage  leaves  are  large  and  leathery  and 
form  a  rosette,  alternating  with  scale  leaves,  at  the  apex  of  the 


FIG.  244.     Zamia,  a  cycad  common  in  southern  Florida,  with  strobilus  of 
megasporophylls. — H.  O.  Hanson. 

stem.  The  bases  of  these  leaves  form  an  armor-like  plate  over 
the  surface  of  the  stems.  In  certain  genera,  the  young  leaves 
are  coiled  as  in  the  ferns. 

(a)  The  Sporophylls  and  Sporangia  of  the  Cycads. — The  spor- 
angia are  borne  on  more  or  less  modified  leaves  arranged  in 
large  strobili  at  the  top  of  the  stem,  though  rarely  on  the  apex. 
Unlike  preceding  cases  these  strobili  are  of  two  kinds,  the  one 
bearing  only  megasporangia,  the  other  microsporangia.  Further- 
more these  two  kinds  of  strobili  are  on  different  plants.  The 
sporangia  are  either  scattered  over  the  sporophyll  or  arranged  in 


DEVELOPMENT   OF   PLANTS 


355 


groups  suggestive  of  the  sori  of  the  ferns,  in  some  forms  even 
showing  a  rudimentary  annulus.  There  is  a  considerable  vari- 
ation in  the  form  of  the  sporophylls  and  the  distribution  of  the 
sporangia.  For  example,  in  Cycas,  the  megasporophylls  are 
loosely  associated  and  only  slightly  modified,  the  sporangia  being 
developed  on  their  margins  (Fig.  245,  3^).  The  microsporo- 


3B 


FIG.  245.  Sporophylls  and  sporangia  of  the  Cycads:  2,  strobilus  of  Zamia. 
2 A,  cross-section  of  strobilus  of  Zamia,  showing  arrangement  of  microsporo- 
phylls.  2JB,  microsporophyll  enlarged,  showing  sporangia  arranged  in  sori. 
2C,  sorus  of  three  sporangia  that  have  opened.  2D,  microsporophyll  of 
Cycas.  3,  cross-section  of  a  strobilus  of  Zamia,  showing  arrangement  of 
megasporophylls.  3-4,  megasporophyll  enlarged  with  two  sporangia.  The 
one  on  the  right  shown  in  section;  ms,  megaspore;  i,  integument;  sp,  spor- 
angium; m,  micropyle.  3$,  megasporophyll  of  Cycas  with  laterally  ar- 
ranged megasporangia. — H.  O.  Hanson. 

phylls  are  small  and  more  compactly  arranged,  the  sporangia 
being  associated  in  sori  on  the  lower  surface  of  the  sporophylls 
(Fig.  245,  2D).  In  Zamia,  the  strobili  and  sporophylls  are  quite 
suggestive  of  Equisetum,  the  sporangia  being  developed  on  the 
inner  side  of  shield-like  sporophylls  (Fig.  245,  2-3^.). 

The  microspores  originate  by  the  division  of  certain  cells  just 
beneath  the  epidermis  and  are  discharged  from  the  sporangium 
very  much  as  in  the  Ophioglossales.  The  megasporangia, 


356 


THE   MEGASPORANGIUM 


however,  present  several  new  departures  that  must  be  borne 
in  mind.  A  rather  thick  coat  or  integument  covers  the  sporan- 
gium save  for  a  small  opening  called  the  micropyle  (Fig.  245,  3^4). 
The  integument  will  be  a  feature  in  all  the  succeeding  groups  and 
it  has  been  compared  to  a  highly  modified  indusium.  It  origi- 
nates, however,  from  the  base  of  the  sporangium  rather  than 
from  the  epidermis  as  in  the  common  ferns.  As  in  Selaginella, 
only  one  of  the  mother  cells  takes  part  in  the  formation  of  spores 
and  but  one  megaspore  (instead  of  four)  is  formed.  This  mega- 
spore  does  not  escape  from  the  sporangium,  owing  to  the  large 


FIG.  246.  Section  of  a  megasporangium  of  Zamia,  showing  the  female 
gametophyte  just  before  fertilization:  i,  integument  differentiating  into  a 
fleshy  outer  and  hard  inner  coat;  m,  micropyle;  p,  cavity  in  which  three 
microspores  are  shown  germinating;  g,  female  gametophyte  which  has  de- 
veloped from  the  single  megaspore.  At  the  micropylar  end  of  the  gameto- 
phyte are  two  archegonia  opening  into  the  archegonial  chamber,  ar.  Two 
neck  cells  lead  to  large  female  gamete,  o;  sp,  remains  of  the  sporangium  not 
yet  consumed  by  the  growth  of  the  gametophyte.  At  the  right  a  few  of  the 
jacket  cells  which  supply  the  female  gamete  with  food  are  enlarged. — After 
Webber. 

number  of  cells  surrounding  it  and  consequently  it  does  not  form 
a  thick  protective  coat.  It  has  been  suggested  that  these  cells 
enveloping  the  megaspore  are  due  to  the  transformation  of  the 
spore  mother  cells,  which  make  up  the  bulk  of  the  fern  sporan- 


DEVELOPMENT   OF   PLANTS  357 

gium,  into  nourishing  cells.  This  structure  of  the  megasporan- 
gium  is  characteristic  of  all  the  spermatophyta  and  the  arrange- 
ment is  of  decided  advantage  since  it  provides  the  megaspore 
during  its  formation  and  germination  with  an  abundance  of 
food  and  moisture. 

(b)  The  Gametophyte  of  the  Eycads.—The  megaspore  germi- 
nates as  in  Selaginella.  Numerous  free  nuclei  are  first  formed 
that  are  arranged  around  the  walls  of  the  spore  and  by  the 
later  formation  of  walls  a  tissue  is  developed  that  gradually 
fills  the  enlarging  spore.  Archegonia  now  appear  at  the  micro- 
pylar  end  of  this  gametophyte  and  become  sunken  in  pits,  called 
the  archegonial  chamber,  owing  to  a  continued  growth  of  the 
adjacent  cells  of  the  gametophyte  (Fig.  246).  While  this  female 
gametophyte  is  very  suggestive  of  the  one  noted  in  Selaginella 
it  should  be  observed  that  the  archegonia  are  more  simple  in 
structure,  usually  possessing  but  two  neck  cells  and  that  the 
central  canal  cells  of  the  neck  are  entirely  lacking.  The  female 
gamete  is  also  much  larger  (Fig.  246,  0)  than  any  heretofore 
noticed  and  it  is  nourished  by  specialized  cells,  the  so-called 
jacket  cells,  that  surround  it  and  supply  it  through  minute  pores 
with  an  abundance  of  food. 

It  is  evident  that  the  buried  position  of  the  archegonia  must 
necessitate  some  new  departure  in  order  to  bring  the  male  ga- 
metes to  the  archegonia.  The  microspores  have  already  begun 
their  germination  at  the  time  when  they  are  discharged  from  the 
sporangium  and  being  very  light,  they  are  carried  by  the  winds 
to  strobili  containing  megasporangia.  When  the  megasporo- 
phylls  are  mature,  they  spread  apart  slightly,  permitting  the 
microspores  to  rattle  down  to  the  sporangia  where  they  are 
caught,  at  least  in  some  forms,  by  a  mucilaginous  secretion 
from  the  micropyle.  This  substance  in  drying  contracts  and 
pulls  the  spores  in  large  numbers  through  the  micropyle  into 
a  chamber  formed  in  the  upper  part  of  the  sporangium  (Fig. 
246,  p).  As  has  been  stated,  the  microspore  begins  to  germinate 
in  its  own  sporangium  and  by  the  time  that  it  reaches  the  cavity 
of  the  megasporangium  its  nucleus  has  already  divided  twice, 
forming  a  rudimentary  male  gametophyte  of  three  cells  that  may 


358 


MALE   GAMETOPHYTE   OF   ZAMIA 


be  compared  with  that  of  Selaginella  (Fig.  247,  A);  the  cell  (g) 
representing  the  small  cell,  and  the  cell  (a)  corresponding  to  the 
large  antheridial  cell  of  the  gametophyte  of  Selaginella  (Fig.  243, 
i),  while  the  cell  (/)  called  the  tube  cell,  represents  a  new  depar- 
ture in  the  evolution  of  the  male  gametophyte.  Thejtube  cell 


E 


FIG.  247.  Male  gametophyte  of  Zamia:  A,  stage  of  germination  of  the 
microspore  attained  in  the  sporangium.  See  text  for  explanation  of  figures. 
B,  formation  of  tube  for  absorbing  of  food  from  megasporangium.  C,  spore 
end  of  gametophyte  showing  the  antheridial  cell  dividing  into  a  body  cell, 
b,  and  a  single  wall  cell,  w.  D,  the  body  cell  has  divided,  forming  two  cells 
which  become  the  male  gametes.  E,  spore  end  of  male  gametophyte,  show- 
ing the  spirally  ciliated  gametes.- — After  Webber. 

grows  out  into  the  tissues  of  the  megasporangium,  forming  a 
tubular  structure,  often  branching  extensively,  and  absorbs  food 
for  the  nourishment  of  the  antfieridial  cell  (Fig.  247,  B).  This 
latter  cell  finally  divides,  forming  a  rudimentary  antheridium  con- 
sisting of  but  a  single  wall  cell  (w)  and  a  body  cell  (b) .  The  body 


DEVELOPMENT   OF   PLANTS  359 

cell  usually  divides  but  once  forming  two  cells,  in  each  of  which 
are  organized  two  huge  male  gametes  with  spirally  arranged 
cilia  (Fig.  247,  C—E).  Owing  to  the  growth  of  the  numerous  tube 
cells,  the  tissues  above  the  archegonial  chamber  become  dis- 
organized and  absorbed  so  that  the  end  of  the  gametophyte 
containing  the  gametes  bends  into  the  cavity  thus  formed  and 
comes  into  close  proximity  to  the  archegonia  (Fig.  246).  Owing 
to  the  swelling  of  the  cells  (g)  and  (w)  assisted  also  by  the  accu- 
mulation of  fluids  in  the  tube,  the  male  gametophyte  is  ruptured 
at  its  basal  or  spore  end  and  the  male  gametes  are  discharged  into 
the  archegonial  chamber  where  they  swim  about  in  the  fluids 
probably  emitted  from  the  tubes  at  the  time  of  their  discharge. 
This  fluid  has  been  observed  by  Chamberlain  to  cause  a  shrinking 
of  the  neck  cells  in  one  species  of  Dioon  and  so  we  see  how  the 
male  gametes  may  enter  the  archegonia. 

The  most  noticeable  departure  in  the  sexual  generation  of 
the  cycads  is  the  very  considerable  reduction  of  the  male  game- 
tophyte which  consists  of  a  delicate  tube  containing  a  few  cells 
from  one  of  which  the  male  gametes  are  formed  as  in  the  ferns. 
You  also  notice  that  the  gametophytes  have  become  entirely 
parasitic  upon  the  sporophyte,  the  male  gametophyte  at  first  upon 
the  sporophyte  which  bore  it  and  later  upon  the  megasporangium. 
Attention  may  be  called  to  a  suggestive  departure  in  the  develop- 
ment of  the  female  gametophyte  that  has  been  observed  in  one 
of  the  cycads.  In  case  fertilization  is  not  effected,  the  gameto- 
phyte ruptures  the  sporangium  and  projects  as  a  green  tissue,  as 
in  certain  heterosporous  ferns. 

(c)  Development  of  the  Sporophyte. — The  germination  of  the 
gametospore  differs  in  two  important  respects  from  the  ferns. 
Its  nucleus  gives  rise  by  repeated  divisions  to  numerous  nuclei 
which  arrange  themselves  around  the  walls  of  the  gametospore, 
becoming  especially  numerous  at  its  lower  end,  and  finally  form 
cell  walls  (Fig.  248,  A).  This  structure  is  called  the  pro-embryo. 
From  the  lower  cells  of  the  pro-embryo  is  organized  a  massive 
suspensor  (Fig.  248,  B),  which  pushes  the  outermost  cells  of  the 
pro-embryo  deep  into  the  tissues  of  the  gametophyte  where  they 
develop  the  young  sporophyte  or  embryo,  which  consists  of  a 


360 


THE  EMBRYO  OF  CYCADS 


rudimentary  root,  stem  and  two  cotyledons  (Fig.  248,  C).  In 
the  ferns  the  gametospore  divided  into  two  cells  which  by  further 
division  formed  the  sporophyte.  The  second  departure  now  ap- 
pears. The  sporophyte  or  embryo  ceases  to  grow  and  it  is 
admirably  protected  during  its  resting  period  by  the  integument 
which  has  become  differentiated  into  a  stony  middle  layer  and  into 
a  fleshy  inner  and  outer  layer.  This  completed  structure,  con- 


FIG.  248.  Development  of  the  sporophyte  or  embryo  of  Cycas:  A;  sec- 
tion of  archegonium,  showing  a  large  number  of  cells  lining  the  walls  of  the 
germinating  gametospore.  This  is  the  pro-embryo  stage.  B,  later  stage 
in  which  the  pro-embryo  has  developed  a  suspensor,  s,  which  pushes  the 
embryo-forming  cells,  e,  into  the  nourishing  tissues  of  the  gametophyte.  C, 
still  later  development,  showing  the  greatly  coiled  suspensor  and  the  young 
sporophyte  or  embryo,  e,  with  two  cotyledons.  D,  section  of  a  seed  of  Zamia: 
i,  integument  which  is  fleshy  without  and  stone-like  within;  m,  micropyle; 
g,  gametophyte;  e,  dormant  sporophyte  or  embryo. — A-C  after  Treub. 

sisting  of  the  modified  integument,  the  embryo,  and  the  gameto- 
phyte, which  has  increased  in  size  until  it  has  absorbed  all  the 
tissues  of  the  sporangium,  we  call  the  seed  (Fig.  248,  D).  At  this 
stage  of  development,  the  seed  falls  from  the  strobilus  and  may 
remain  in  a  dormant  condition  for  years. 

The  formation  of  the  seed  is  the  most  characteristic  feature 
of  the  Spermatophyta  and  the  most  important  advance  that  is 
to  be  noted  in  the  evolution  of  plant  life.  This  variation  has 
given  the  seed  plants  an  advantage  over  all  other  terrestrial 
groups  and  made  them  the  dominant  plants  upon  the  earth.  Not 


DEVELOPMENT  OF  PLANTS 


only  are  the  male  and  female  gametophytes  better  provided  for 
than  in  Selaginella,  owing  to  their  complete  parasitism  oh  the 
well  developed  sporophytes,  but  the  formation  of  the  young 
sporophyte  or  embryo  is  also  ensured  by  the  abundance  of  food 
placed  at  its  disposal.  The  most  important  advantage,  however, 
possessed  by  the  seed  plant  appears  in  the  modification  of  the 
integument  which  renders  it  impervious  to  gases  and  fluids. 
This  change  so  effectually  seals  up  the  embryo  as  to  protect  it 
for  years  in  some  cases  against  conditions  unfavorable  for  growth. 
The  modification  of  the  integument  was  doubtless  the  chief 
factor  that  led  to  the  seed  habit,  since  it  cuts  off  the  supply  of 


FIG.  249.  Renewal  of  growth  of  the  sporophyte  or  embryo  in  the  seed. 
7,  section  of  the  seed,  showing  the  base  of  the  cotyledons  extending  from  the 
seed,  thus  pushing  out  the  stem  and  root,  the  latter  organ  (shown  in  part) 
curving  down  into  the  ground.  The  free  ends  of  the  cotyledons  are  enlarging 
as  they  absorb  the  food  stored  in  the  gametophyte.  6,  seedling  six  months 
old  with  first  normal  leaf. — After  Sachs. 

oxygen  and  fluids  from  the  sporophyte  or  embryo  and  thus 
stops  its  growth.  When  conditions  are  favorable  for  growth 
(see  page  129),  the  integument  becomes  permeable  to  fluids  and 
gases  and  the  embryo  continues  its  development.  The  cotyle- 
dons remain  in  the  seed  to  absorb  the  food  stored  in  the  gameto- 
phyte, but  their  basal  portions  elongate,  pushing  out  the  root 
which  soon  becomes  established  in  the  soil,  while  the  stem  which 
is  carried  out  with  the  root  extends  up  into  the  air,  and  de- 
24 


362  THE   CONE-BEARING  TREES 

velops  the  leaves  (Fig.  249).  This  renewal  of  growth  of  the 
embryo,  or  sprouting  of  the  seed,  is  often  erroneously  termed  the 
germination  of  the  seed. 

The  maidenhair  tree,  Gink  go  biloba,  a  native  of  China  and 
Japan  and  now  extensively  cultivated  throughout  the  world,  is 
the  sole  survivor  of  another  ancient  order  of  gymnosperms  that 
have  many  points  in  common  with  the  cycads  and  ferns.  The 
tall  shaft  of  the  stem,  wide-spreading  branches  and  the  arrange- 
ment and  character  of  the  tissues  are  more  suggestive  of  the  next 
order,  the  cone-bearing  trees.  The  leaves,  however,  are  fern- 
like  in  appearance  and  are  deciduous,  unlike  most  of  the  Gymno- 
spermae.  The  arrangement  of  the  sporangia,  the  development 
of  the  gametophytes,  and  fertilization  are  of  the  same  nature 
as  in  the  cycads.  It  is  noteworthy  that  fertilization  may  be 
effected  after  the  megasporangia  have  fallen,  thus  approaching 
the  condition  seen  in  Selaginella. 

121.  Order  b.  Finales  or  Cone-bearing  Trees. — This  order  is 
by  far  the  largest  group  of  gymnosperms  and  includes  our  well- 
known  evergreen  or  cone-bearing  trees.  They  are  largely  con- 
fined to  temperate  regions  and  are  more  numerous  in  the  northern 
than  in  the  southern  hemisphere.  The  shores  of  the  Pacific  are 
especially  favorable  for  their  development  and  on  our  western 
coast  they  attain  dimensions  equalled  only  by  the  giant  Eucalyp- 
tus of  Australia.  Though  less  ancient  in  origin  than  the  cycads, 
they  have  declined  in  the  same  way  as  the  latter  order  and  only  a 
remnant  of  this  very  extensive  group  survives  today.  However, 
their  variations  have  been  more  beneficial  than  in  the  case  of  the 
cycads  and  as  a  result  nearly  300  species  are  adapted  to  present 
conditions  upon  the  earth  and  constitute  an  important  part,  and 
commercially  the  most  valuable  part  of  our  forest  vegetation. 

(a)  Structural  and  Adaptive  Features  of  the  Pinoles. — One  of 
the  variations  that  is  of  great  benefit  to  many  of  these  plants 
is  their  habit  of  forming  buds  (see  page  71),  which  appear  in  this 
order  in  more  perfect  form  than  any  of  the  preceding.  By 
means  of  the  scale  leaves  of  the  buds  all  the  organs  that  are  to 
be  produced  each  season  are  protected  against  drying  winds. 
The  terminal  bud  of  the  tree  is  frequently  the  most  vigorous  and 


DEVELOPMENT  OF  PLANTS 


363 


as  a  result  spire-like  stems  with  very  symmetrically  arranged 
branches  are  often  formed.  The  growth  of  the  Finales  is  usually 
prolonged  and  vigorous,  the  age  of  some  of  the  giant  trees  of 
California  being  estimated  at  over  5,000  years,  and  they  may 
attain  the  height  of  over  300  feet  and  a  diameter  of  30  feet.  The 
elongation  of  the  stem  is  effected  by  a  group  of  actively  dividing 
cells  at  the  apex,  while  the  increase  of  girth  is  brought  about 
by  the  activities  of  a  cambium,  as  in  the  dicotyledons  (see  page 
89).  In  fact,  all  the  features  seen  in  the  cross-section  of  the 
stem  as  the  cork,  cortex,  phloem,  cambium,  xylem  with  its  an- 
nual rings  of  growth  and  narrow  rays  and  the  pith  are  sugges- 
tive of  the  dicotyledons,  save  that  the  tissues  are  more  simple. 
This  is  particularly  noticeable  in  the  xylem,  which  is  composed 


FIG.  250.  A,  radial  section  of  the  xylem  of  Pinus — t,  tracheids;  p,  border 
pores  or  thin  places  in  cell  wall  to  promote  transfer  of  fluids;  ra,  a  wood 
ray  of  two  cells  accompanied  by  tracheids,  mt.  B,  tangential  section,  show- 
ing rays,  one  cell  broad  and  from  three  to  nine  cells  high.  p,  bordered 
pores.  C,  cross-section  of  the  outer  cells  of  pine  leaf,  showing  heavy  epi- 
dermal cells,  e;  well-developed  cuticle,  c;  sunken  stoma,  s;  strengthening 
stereome  cells,  st,  and  chlorenchyma,  m. 

of  very  regularly  formed  tracheids  with  peculiar  bordered  pores 
on  their  radial  walls  to  promote  the  rapid  transfer  of  fluids  to 
and  from  the  wood  rays  (Fig.  -250,  A,  B).  The  character- 
istic uniform  and  even  grain  of  pine,  spruce,  etc.,  is  due  to  the 
simplicity  of  the  tracheids  which  compose  the  wood.  This  struc- 


364  STRUCTURE   OF  THE   FINALES 

ture  of  the  wood  explains  the  ease  with  which  the  timber  is 
worked  and  the  common  name  of  "soft  wood"  that  is  applied 
to  it.  The  leaves  are  xerophytic  in  character  and  usually  rather 
small  and  often  needle-like.  The  strongly  thickened  epidermis 
is  strengthened  with  strands  of  stereome  that  give  them  a  leathery 
character  (Fig.  250,  C).  The  stomata  are  sunken  and  the 
centrally-placed  vascular  bundles  communicate  with  the  rather 
irregular  chlorenchyma  cells  by  means  of  tracheids,  and  thin- 
walled  parenchyma  cells  instead  of  small  veins  as  in  the  higher 
plants.  The  leaves  in  the  majority  of  the  genera  remain  on  the 
branches  for  several  years,  and  for  this  reason  the  members  of 
the  Finales  are  commonly  called  "evergreens." 

These  plants  generally  flourish  in  localities  where  there  is  an 
abundance  of  moisture  and  at  first  sight  the  modifications  of  the 
leaves  noted  above  are  not  in  harmony  with  such  surroundings, 
being  decidedly  characteristic  of  plants  growing  in  arid  regions. 
The  minute  tracheids  which  constitute  the  conducting  system  of 
the  Finales  cannot,  however,  transport  the  fluids  absorbed  from 
the  soil  as  rapidly  as  the  large  cells  noted  among  the  dicotyledons. 
Consequently  smaller  amounts  of  water  are  placed  at  the  disposal 
of  the  leaves,  and  were  it  not  for  the  fact  that  they  are  admirably 
adapted  to  lessen  transpiration  these  plants  would  doubtless  have 
disappeared  from  the  earth  long  ago.  The  ability  of  the  leaves 
to  diminish  the  loss  of  water  may  also  account  for  the  common 
occurrence  of  the  cone-bearing  trees  in  northern  and  mountainous 
districts  where  the  soils  are  cold  and  root  absorption  is  therefore 
lessened  and  where  the  vegetation  is  especially  exposed  to  the 
drying  winds  of  winter  (see  page  38). 

Nearly  all  the  Finales  are  characterized  by  the  presence  of 
resin  passages  or  ducts  in  the  stems  and  leaves.  Whenever  the 
stems  are  cut  or  injured  these  passages  pour  out  a  thick  resinous 
liquid  that  effectually  heals  the  wound.  This  doubtless  explains 
in  part  the  freedom  of  these  trees  from  the  attack  of  insects  and 
wood-destroying  fungi.  It  may  possibly  account  for  the  inabil- 
ity of  many  of  these  trees  to  sprout  from  the  stump  when  cut. 
Hardwood  forests  will  very  generally  perpetuate  themselves  by 
means  of  sprouts  that  arise  from  the  stumps,  but  our  evergreen 


DEVELOPMENT   OF   PLANTS 


365 


forests  are  rapidly  disappearing,  because  with  few  exceptions  the 
trees  are  able  only  to  develop  from  seeds.  Possibly  the  rapid 
covering  of  living  cells  of  the  stump  by  the  resinous  substances 
excludes  the  atmosphere  so  effectually  as  to  prevent  any  further 
growth  (page  75).  This  resinous  liquid  is  obtained  in  large 
quantities  from  certain  species  of  pine  by  removing  the  bark  and 
a  little  of  the  wood  from  the  side  of  the  trees  when  the  liquid 
exudes  and  is  caught  in  a  pan  placed  at  the  bottom  of  the  cut 
surface  or  in  a  pocket  made  in  the  tree.  This  substance  is  re- 
moved from  time  to  time  and  distilled,  yielding  turpentine  and 
resin.  Balsam  is  a  resin  obtained  from  the  balsam  fir  (Abies), 
(b)  Thz  Sporangia  of  the  Pinoles. — The  sporangia  resemble 
those  of  the  cycads  in  origin  and  structure  and  are  developed 
upon  sporophylls  that  are  usually  grouped  in  strobili  (Figs.  251, 


FIG.  251.  Microsporophylls  of  the  pine:  I,  branch  of  pitch  pine  with 
leaves  spirally  arranged  in  fascicles  of  three  and  also  with  several  strobili, 
st.  At  tip  of  branch  the  bud,  b,  is  shown  elongating  and  bearing  numerous 
fascicles  of  leaves  that  are  still  enclosed  by  paper  bracts.  2,  surface  and  side 
view  of  microsporophyll,  showing  arrangement  of  sporangia,  sp.  2a,  micro- 
sporophyll  of  juniper  bearing  three  sporangia. — H.  O.  Hanson. 

252).  The  microsporangia  are  formed  on  rather  small,  delicate 
and  often  brightly  colored  sporophylls  that  perish  as  soon  as  the 
spores  are  scattered  (Fig.  251,  2).  The  microsporophylls  bear 
one  or  several  sporangia  on  their  under  surface  and  are  hygro- 


366 


SPOROPHYLLS   OF   PINALES 


scopic,  curving  away  from  each  other  when  the  sporangia  are 
ripe,  thus  permitting  the  opening  of  the  sporangia  and  the  gradual 
distribution  of  the  spores  by  the  wind  and  closing  when  mois- 
tened to  protect  the  spores  against  wetting.  In  some  of  these 
genera  the  outer  coat  of  the  microspore  is  lifted  away  from  the 
inner  coat  in  such  a  way  as  to  form  two  sac-like  outgrowths 
which  render  them  more  buoyant  and  adapted  to  distribution  by 
the  wind  (Fig.  254).  The  miscrospores  are  produced  in  such 
enormous  numbers  that  the  ground  in  the  vicinity  of  the  ever- 


FIG.  252.  Strobili  of  megasporophylls:  I,  a  branch  that  has  developed 
in  the  spring  from  a  bud.  The  fascicles  of  leaves  are  just  emerging  from 
their  papery  bracts  and  at  the  right  is  shown  a  strobilus,  st,  consisting  of 
minute  spine-tipped  scales.  2,  growth  of  strobilus  shown  in  I,  twelve  months 
later.  3,  appearance  of  strobilus  shown  in  2,  six  months  later. — H.  O.  Hanson. 

green  forests  is  often  covered  with  the  yellow  spores,  the  so- 
called  "sulphur  snow."  This  extravagant  formation  of  micro- 
spores  is  a  necessity  owing  to  the  small  chance  of  any  one  of  the 
microspores  being  carried  to  the  megasporophylls  and  so  ensur- 
ing fertilization.  Were  it  not  for  the  fact  that  the  microspores 
need  be  carried  only  comparatively  short  distances,  this  order 


DEVELOPMENT  OF  PLANTS  367 

would  doubtless  become  extinct.  The  gymnosperms,  however, 
live  in  colonies  and  the  strobili  of  mega-  and  micro-sporophylls 
usually  occur  upon  the  same  plant,  thus  necessitating  but  a  short 
transfer  of  microspores.  The  megasporophylls  usually  endure 
for  a  year  or  more  and  become  quite  large,  forming  the  leathery 
or  woody  scales  of  the  cone-  or  strobilus  (Fig.  252).  The  mega- 
sporangia  are  usually  developed  on  the  upper  side  of  the  sporo- 
phyll  (Fig.  253,  A),  the  number  formed  varying  in  the  different 
genera.  In  the  majority  of  the  Pinales,  the  sporangia  appear 
upon  curious  outgrowths  which  become  large  and  form  the 
conspicuous  scales  of  the  cone.  This  structure  is  generally 
regarded  as  a  greatly  modified  branch  or  shoot  with  which  two 
sporophylls  are  fused.  It  generally  arises  from  the  axil  of  a 
bract  just  below  it  and  with  which  it  may  be  more  or  less  united — 
see  the  spruces,  pines,  firs,  etc.  (Fig.  253,  B).  The  origin  and 
structure  of  the  megasporangium  and  megaspore  are  essentially 
as  noted  in  the  cycads.  There  is  only  slight  indication,  however, 
in  a  few  of  the  forms  of  the  development  of  a  cavity  for  the 
reception  of  the  microspores.  Several  mother  cells  may  form 
megaspores,  but  only  one  of  these  megaspores  ever  develops  and 
germinates. 

(c)  The  Gametophyte  of  the  Pinales  and  Fertilization. — The 
female  gametophyte  is  formed  as  previously  noted  in  the  cycads. 
By  the  repeated  divisions  of  the  nucleus  of  the  megaspore  numer- 
ous free  nuclei  are  formed  that  become  arranged  around  the  walls 
of  the  megaspore.  Later  cell  walls  are  developed  about  these 
nuclei,  and  by  further  division  the  entire  space  within  the  enlarg- 
ing spore  is  filled  with  tissue  (Fig.  253,  C).  A  few  or  a  large 
number  of  archegonia  are  developed  at  the  micropylar  end  of 
the  gametophyte.  The  archegonium  consists  of  two  or  several 
neck  cells,  and  a  large  female  gamete  which  is  surrounded  by 
nourishing  cells  as  in  the  cycads.  It  is  noteworthy  in  a  few  of  the 
forms  that  the  archegonia  begin  to  appear  very  early  in  this 
growth  outlined  above  and  that  the  bulk  of  the  female  gameto- 
phyte is  developed  subsequent  to  their  origin.  The  possible 
significance  of  this  will  be  more  apparent  in  the  Angiosperms, 
where  we  will  see  the  female  gamete  arising  with  the  third  division 


368 


GAMETOPHYTE   OF   FINALES 


of  the  nucleus  of  the  megaspore,  the  gametophytic  tissue  being 
developed  at  a  later  period.  You  have  noticed  that  there  is  no 
chamber  formed  above  the  archegonia,  and  it  would  be  a  natural 
inference  that  the  male  gametophyte  must  be  of  a  somewhat 
different  character  from  that  of  the  cycads  in  order  to  met  this 


FIG.  253.  Megasporangia  of  pine:  A,  scale-like  outgrowth  of  sporophyll 
bearing  two  sporangia.  B,  section  of  A,  taken  through  one  of  the  sporangia, 
showing  a  small  bract,  s,  and  large  scale-like  outgrowth  which  bears  the 
sporangia,  sp.  The  sporangium  is  surrounded  t>y  an  integument,  i,  and  con- 
tains a  single  megaspore,  m,  as  in  the  Cycads.  me,  microspores  that  have 
been  drawn  through  the  micropyle  to  the  sporangium,  into  which  their  tube 
cells  are  growing.  C,  section  of  megasporangium  enlarged  and  diagrammatic  , 
showing  the  female  gametophyte  at  the  time  of  fertilization — g,  female  garnet 
ophyte,  showing  two  archegonia  with  large  female  ^gametes,  fg.  At  the  left 
the  tube  of  the  microspore,  me,  has  entered  the  neck  of  the  archegonium  and 
the  two  male  gametes  have  been  discharged  into  the  large  sac  of  the  female 
gamete.  On  the  right  the  tube  cell  with  male  gametes  is  seen  approaching 
the  neck  of  the  archegonium^  i,  integument;  sp,  remains  of  sporangium 
not  disorganized  by  the  gametophyte;  t  shows  the  disorganization  caused  by 
the  tube  cells  as  they  grow  towards  the  archegonia. 


DEVELOPMENT   OF   PLANTS 


369 


new  departure.  The  microspores  which  have  already  begun  to 
germinate  when  discharged  from  their  sporangia  are  carried  by 
the  wind  to  the  megasporophylls  which  are  slightly  spread  apart 
at  this  time,  permitting  the  microspores  to  rattle  down  to  the 
megasporangia.  The  microspores  fall  into  the  micropyle  either 
by  reason  of  the  position  and  construction  of  the  megasporangia, 
which  may  be  so  placed  that  the  spores  naturally  roll  down  into 
the  micropyle,  or  the  microspores  may  be  drawn  through  the 
micropyle  by  mucilaginous  excretions  as  in  the  cycads.  The 


8 


FIG.  254.     Male  gametophyte  of  the  pine:  6,  section  of  microspore,  show- 
ing the  two  air  sacs,  s,  formed  by  the  lifting  up  of  the  outer  wall  of  the  spore. 

8,  stage  of  germination  of  the  spore  at  time  of  discharge  from  its  sporangium 
— t,  tube  cell;  a,  antheridial  cell,  above  which  are  seen  two  black  lines,  the 
remains  of  cells  formed  by  earlier  division  of  the  nucleus  of  the  microspore, 

9,  continuation  of  germination  of  the  microspore  after  reaching  the  mega- 
sporangium — /,  tube  cell  forming  a  branching  tube  that  disorganizes  and  ab- 
sorbs the  cells  of  the  megasporangium.     The  antheridial  cell  (a)  of  8  has 
divided  into  a  wall  cell,  w,  and  a  body  cell,  b.     10,  end  of  tube  cell  as  it  ap- 
proaches the  female  gamete — m,  m,  male  gametes.     The  nuclei  of  the  tube 
cell  and  wall  cell  are  also  seen  (somewhat  disorganized)  in  the  end  of  the  tube. 
— After  Coulter  and  Chamberlain. 

early  stages  in  the  germination  of  the  microspore  are  essentially 
as  in  Cycas,  though  generally  more  reduced.  The  microspore  on 
reaching  the  megasporangium  has  germinated,  forming  a  tube 


370  MALE    GAMETOPHYTE  OF  FINALES 

cell  and  an  antheridial  cell,  but  the  one  or  two  cells,  which  corre- 
spond to  the  cell  (g)  of  the  gametophyte  noted  in  the  cycads,  are 
quickly  disorganized  and  appear  as  faint  lines  (Fig.  254,  8). 
In  some  forms  several  of  these  prothallial  cells  are  formed  so  that 
the  male  gametophyte  is  more  primitive  in  this  respect  than  the 
cycads.  On  the  other  hand  other  groups  do  not  apparently 
form  a  prothallial  cell  at  all.  So  the  tendency  is  towards  the 
elimination  of  these  cells  because  they  are  no  longer  of  service 
in  setting  free  the  gametes.  The  tube  cell  develops  as  in  the 
cycads,  but  it  is  directed  in  its  growth  so  that  one  of  its  branches 
finally  reaches  one  of  the  archegonia,  when  it  pushes  aside  the , 
neck  cells  and  fuses  with  the  cell  membrane  of  the  large  female 
gamete  (Figs.  253,  C\  255,  A).  In  the  meantime  the  antheridial 
cell  has  divided  into  a  wall  cell  and  body  cell  (Fig.  254,  9),  and 
two  motionless  male  cells  are  formed  from  the  body  cell.  It  is 
to  be  noted  here  that  the  body  cell  does  not  divide,  forming  cells 
in  which  male  gametes  are  organized,  as  in  all  preceding  cases. 
It  forms  two  free  nuclei  and  these  with  their  associated  cytoplasm 
function  as  gametes.  It  is  now  apparent  why  the  tube  cell 
extends  to  the  female  gamete  and  also  why  the  archegonial 
chamber  is  not  developed.  The  male  gametes  and  the  wall  cell, 
now  quite  unattached,  are  carried  down  to  the  end  of  the  tube 
(Fig.  254,  10)  by  the  cytoplasmic  currents  and  pass  into  the  cyto- 
plasm of  the  female  gamete  through  an  opening  that  is  formed 
in  the  end  of  the  tube.  Chemical  attraction  now  draws  one  of 
the  male  gametes  to  the  nucleus  of  the  female  and  their  fusion 
results  in  the  formation  of  the  gametospore.  The  remaining 
cells  of  the  male  gametophyte  are  apparently  disorganized 
(Fig.  255,  A).  Several  instances  have  been  reported  where  the 
male  cells  differ  in  size,  indicating  the  loss  of  function  in  one  of 
these  cells.  On  the  other  hand  in  those  groups  where  the 
archegonia  are  closely  associated  and  where,  therefore,  the  tube 
cell  may  spread  over  more  than  one  archegonium,  no  such  reduc- 
tion appears.  The  functions  performed  by  the  tube  cells  in  the 
Finales  will  be  a  constant  characteristic  of  all  the  members  of 
the  next  subdivision  or  Angiospermae,  and  it  should  be  noted  that 
this  important  modification  of  the  male  gametophyte  probably  * 


DEVELOPMENT   OF   PLANTS 


371 


arose,  at  first  as  an  absorbing  organ  to  supply  the  antheridial  cell 
with  food,  as  is  the  case  in  the  cycads.  In  the  more  advanced 
types  the  archegonia  became  more  effectually  enclosed  in  the 
sporangial  tissues  and  the  tube  cell  assumed  in  addition  the 
function  of  a  conduit  for  the  male  gametes  which  lost  their 


FIG.  255.  Development  of  the  pro-embryo  of  pine:  A,  section  of  an  ar- 
chegonium  at  time  of  fertilization.  One  of  the  male  gametes,  cf,  is  seen 
fusing  with  the  female,  9.  The  second  male  gamete,  cf,  the  tube  nucleus, 
and  the  wall  cell  are  also  shown  near  the  neck  of  the  archegonium.  B,  the 
gametospore  has  germinated,  forming  four  cells,  which  are  passing  to  the 
upper  end  of  the  sac.  C,  the  cells  shown  in  B,  arranged  at  end  of  sac  and 
in  process  of  division  (only  two  cells  in  this  sectional  view).  D,  later  stage, 
the  pro-embryo  of  four  plates  of  cells.  E,  the  second  plate  or  suspensory 
cells,  s,  of  the  pro-embryo  elongating,  thus  pushing  the  embryo-forming  cells, 
e,  into  the  tissues  of  the  female  gametophyte. — A  after  Ferguson;  B-E  after 
Coulter  and  Chamberlain. 

motility  as  a  result  of  this  new  method  of  transport  to  the 
female  gamete. 

(d)  Development  of  the  Sporophyte. — The  germination  of  the 
gametospore  of  the  pine  will  illustrate  the  more  important 
features  in  the  process.  The  nucleus  of  the  gametospore  divides 
a  varying  number  of  times,  commonly  forming  four  nuclei  (Fig. 
)i  which  pass  to  the  upper  end  of  the  spore  and  arrange 


372 


EMBRYO   OF   FINALES 


themselves  in  a  plate.     By  successive  division  these  cells  are 
increased  until  usually  four  plates  of  four  cells  each  are  formed 


FIG.  256,  Development  of  the  young  sporophyte  or  embryo:  2,  diagram 
of  a  section  of  a  megasporangium,  showing  the  formation  of  elongated  sus- 
pensory cells  and  numerous  embryos,  e;  i,  integument;  sp,  sporangium  nearly 
consumed  by  the  growth  of  the  gametophyte;  g,  gametophyte,  the  central 
portion  of  it  disorganized  by  growth  of  embryos,  e.  $A,  section  of  a  nearly 
mature  embryo;  s,  suspensor;  r,  root  cap;  c,  cotyledons;  st,  stem.  $B,  external 
view  of  embryo. 

which  become  surrounded  by  cell  walls  save  in  the  case  of  the 
lower  plate,  which  is  in  direct  contact  with  the  rich  food  of  the 
spore  (Fig.  255,  C-D).  This  meager  growth  corresponds  to  the 


FIG.  257.  Sectional  view  of  pine  seed:  i,  hard  integument;  g,  gameto- 
phyte, often  called  the  endosperm,  which  has  completely  consumed  the  spor- 
angial  tissues.  The  embryo  consists  of  a  root,  r,  ensheathed  in  a  large  root  cap, 
cotyledons,  c,  and  stem,  s. 


DEVELOPMENT   OF   PLANTS  373 

pro-embryo  of  Cycas  (Fig.  248,  A).  In  this  connection  it  should 
be  noticed  that  only  a  portion  of  the  gametospore,  as  in  the 
cycads,  is  used  in  forming  the  embryo.  In  all  the  preceding 
groups,  as  the  mosses,  ferns,  etc.,  the  gametospore  in  germinating 
behaves  as  an  ordinary  cell,  dividing  into  two  cells  which  con- 
tinue the  process.  In  the  Pinales  the  larger  portion  of  the 
gametospore  serves  as  a  storehouse  for  food,  cell  formation  being 
confined  to  the  upper  end  of  it  (Fig.  255,  B,  C).  With  one  ex- 
ception this  is  the  only  group  characterized  by  this  peculiar  ger- 
mination which  is  more  suggestive  of  the  growth  of  the  animal 
egg  where  a  large  portion  of  the  egg  is  reserve  food  for  the  nourish- 
ment of  the  embryo.  The  nourishment  of  the  female  gamete 
by  the  jacket  cells  is  also  suggestive  of  certain  animals  in  which 
the  egg  cells  are  formed  in  a  similar  manner.  The  embryo  is 
developed  from  the  uppermost  plate  of  cells  which  are  pushed  into 
the  cells  of  the  gametophyte  by  the  elongation  of  the  suspensory 
cells  just  below  them  (Fig.  255,  E).  Each  of  these  four  cells 
may  form  an  embryo  (Fig.  256,  2)  or  they  function  collectively 
in  this  growth.  The  result  is  the  same  in  all  cases,  rarely  more 
than  one  embryo  being  found  in  a  seed.  The  mature  embryo 
consists  of  a  stem  bearing  two  or  several  laterally-placed  coty- 
ledons and  a  root  with  root  cap  (Fig.  256,  3).  Attending  this 
formation  of  the  embryo,  pronounced  changes  occur  in  the 
sporangium.  It  steadily  increases  in  size  and  the  integuments 
become  modified  into  a  hard  coat,  or  the  outer  layer  may  be 
pulpy,  as  in  some  of  the  cycads.  The  gametophyte  also  increases 
in  size,  and  as  the  embryo  matures  it  absorbs  all  the  tissues  of  the 
sporangium,  thus  filling  the  space  within  the  integument  (Fig. 
257).  The  cells  of  the  female  gametophyte  are  often  called  the 
"endosperm."  By  these  growths  the  seed  is  formed  and  pre- 
pared for  its  dormant  state,  as  in  the  preceding  order.  In  some 
cases  the  integument  forms  a  membranous  outgrowth,  which 
assists  in  the  distribution  of  the  seeds  (Fig.  258,  Q.  It  is  inter- 
esting to  note  that  the  seeds  are  so  attached  to  these  wings  that 
they  rotate  in  falling  to  the  ground,  thus  retarding  their  fall  and 
making  possible  a  wider  flight.  The  stimulus  resulting  from  the 
fusion  of  the  gametes  also  produces  extensive  changes  outside  of 


374  THE   SEED   OF   FINALES 

the  sporangia.  The  sporophylls  or  their  outgrowths  often  be- 
come greatly  enlarged,  forming  the  hard,  woody  scales  of  the 
cones  (Fig.  258,  B)',  or  they  may  become  fleshy  and  fuse,  forming 
a  berry-like  fruit,  as  in  the  juniper  (Fig.  260,  7).  When  this 
growth  has  been  completed,  the  woody  sporophylls  or  scales  of 
the  strobilus  dry  out,  and  becoming  hygroscopic,  they  spread 
apart  on  dry  days,  thus  permitting  the  scattering  of  the  seeds 
by  the  winds  (Fig.  258,  A).  Many  of  the  fleshy  fruits  are  eaten 
by  birds  and  the  hard  nut-like  seeds  are  distributed  in  this  way. 


FIG.  258.  A,  mature  strobilus  of  pine  with  open  scales  to  permit  the 
scattering  of  the  seeds.  B,  scale  from  strobilus  showing  the  winged  seeds 
developed  from  the  two  sporangia.  C,  a  seed  with  wing-like  outgrowth, 
as  it  escapes  from  strobilus. 

The  stages  in  the  development  of  the  seed,  outlined  above,  are 
very  much  prolonged  in  many  of  the  pines.  The  microspores  on 
reaching  the  megasporangia,  in  the  spring,  develop  only  a  short 
tube  cell  during  the  first  season  and  not  until  about  the  first  of 
July  of  the  following  season  are  the  male  and  female  gametes 
mature  and  ready  for  fertilization.  The  seeds  are  matured  during 
the  following  season,  over  two  years  after  the  appearance  of 
the  strobilus.  Accordingly,  three  stages  in  the  development 
of  the  strobilus  of  megasporophylls  may  be  seen  on  certain 
species  of  pine  in  the  early  summer — very  small  ones  that  received 
the  microspores  in  the  spring,  larger  ones  in  which  fertilization 
and  the  formation  of  the  embryo  is  being  effected  and  older  stro- 
bili  in  which  the  seeds  are  approaching  maturity  (Fig.  252). 
When  the  conditions  are  favorable  the  embryo  renews  its  growth. 


DEVELOPMENT   OF   PLANTS  375 

The  root  is  pushed  out  through  the  micropyle  and  bends  down 
into  the  soil,  the  hard  integument  often  being  cracked  open  by 
the  swelling  of  the  cells.  The  cotyledons  remain  within  the  seed 
until  they  have  absorbed  all  the  food  from  the  gametophyte, 
when  they  are  withdrawn  and  become  erect  (Fig.  259). 


FIG.  259.  Renewal  of  growth  of  the  embryo.  At  the  left  a  seed  has  been 
cut  across  to  show  the  relation  of  parts  during  the  early  growth  The  in- 
tegument has  been  ruptured  by  the  swelling  of  the  seed  and  the  protrusion 
of  the  root  which  is  curving  down  into  the  soil.  The  cotyledons,  c,  remain 
in  the  seed  absorbing  the  food  from  the  gametophyte  or  endosperm,  g.  At 
the  right  a  later  growth  with  the  cotyledons  partially  withdrawn  from  the 
seed  after  the  absorption  of  its  food. 

(e)  The  Principal  Genera  of  Pinoles. — The  more  important 
genera  of  Pinales  may  be  distinguished  as  follows:  Pinus  or 
pine,  leaves  long  and  needle-like,  borne  in  fascicles  on  short  stems 
that  are  quite  concealed  by  papery  sheathing  scales  (Fig.  251,  i). 
Larix  or  larch,  short  needle-like  leaves  clustered  in  tufts  on  short 
lateral  branches.  This  is  the  only  northern  member  of  the  order 
with  deciduous  leaves.  Picea  or  spruce,  leaves  angled  or  four- 
sided,  radiating  from  all  sides  of  stem,  petioles  remaining  on 
branchlet  after  leaves  fall,  thus  causing  the  rough  appearance  of 
the  branchlets  (Fig.  260,  9).  Tsuga  or  hemlock,  leaves  flat  in 
two  rows,  the  petiole  remaining  on  the  stem  after  fall  of  leaf; 


376 


FORMS   OF   FINALES 


the  strobilus  is  ovoid  with  rather  leathery  scales.  The  spruce 
and  hemlock  have  pendent  strobili.  Abies  or  fir,  leaves  flat  in 
two  rows,  the  stems  being  smooth  after  leaf  fall,  strobili  erect  on 
the  branches.  Taxodium  or  bald  cypress,  leaves  flat  in  two  rows 


7 

FIG.  260.  Common  examples  of  the  Finales:  5,  Thuja  or  arbor  vitae. 
6,  Strobilus  of  Chamaecyparis  or  southern  white  cedar.  7,  strobilus  of  Juni- 
perus  or  red  cedar  with  fleshy  scales  fused  into  a  berry-like  fruit.  8,  branch 
of  Taxus  or  yew.  The  seeds  are  produced  singly  in  the  axils  of  leaves  on  short 
lateral  branches  and  nearly  enveloped  by  a  thick  fleshy  cup  that  becomes 
bright  red.  9,  Picea  or  spruce. 

and  deciduous,  strobilus  globose  with  thick,  spirally-arranged  and 
shield-like  scales.  Thuja,  arbor  vitae,  or  northern  white  cedar, 
leaves  scale-like,  closely  appressed  to  the  branches  in  four  rows, 
strobili  ovoid  with  flat  tough  scales  (Fig.  260,  5).  Chamaecyparis 


DEVELOPMENT  OF  PLANTS  377 

or  southern  white  cedar,  leaves  resembling  Thuja,  but  cones 
globose  with  shield-like  opposite  scales  (Fig.  260,  6).  Juniperus 
or  juniper,  pointed  scale-like  leaves,  opposite  or  in  whorls  on 
stem,  scales  of  strobili  becoming  fleshy  and  fusing  to  form  a 
berry-like  fruit  (Fig.  260,  7). 


CHAPTER  X 

SUBDIVISION    2.     ANGIOSPERMAE.     PLANTS    WITH    ENCLOSED 

SEEDS 

122.  Origin   of   the   Angiospermae. — These   plants,   like   the 
Gymnospermae,  produce  seeds,  but  they  differ  so  essentially  from 
the  latter  group  in  the  character  and  structure  of  the  sporophyte 
and  gametophyte  as  to  justify  their  separation  into  a  distinct 
division.     They  are  retained  here  as  a  group  coordinate  with 
the  gymnosperms  out  of  deference  to  common  usage.     It  has 
been  noticed  that  the  seed  habit  arose  quite  independently  of 
the  Spermatophyta  (see  page  351)  and  it  appears  probable  that 
the  Angiospermae  represent  an  independent  line  of  development. 
Though  the  most  recently  evolved  plants,  their  origin  is  uncertain 
and  the  meager  evidence  points  to  their  derivation  from  a  stock 
related  to  the  more  primative  Filicales  rather  than  to  the  Gym- 
nospermae.    The  variations  of  this  modern  group  of  plants  have 
been  many  and  so  successful  that  they  have  crowded  out  many 
of  the  lower  forms  and  become  the  dominant  plants  upon  the 
earth.     They  exceed  all  other  groups  combined  in  variety  and 
number  of  forms,  approximately,   125,000  species — certainly  a 
striking  contrast  to  the  other  vascular  plants  which  comprise 
about  450  gymnosperms  and  4000  pteridophytes.     The  angio- 
sperms  are  adapted  by  their  variations  to  practically  all  con- 
ditions that  will  sustain  life,  ranging  from  aquatics  to  xerophytes, 
from  terrestrials  to  epiphytes  and  from  photosynthetic  to  sapro- 
phytic  and   parasitic  plants.     The  important   features  of  the 
plant  body  have  been  considered  in  the  opening  chapters  of  the 
book. 

123.  The  Sporophylls  of  Angiosperms. — The  flower  is  often 
considered  as  one  of  the  most  characteristic  features  of  the 
angiosperms,  but  this  structure  contains  as  its  essential  organs 
one  or  more  sporophylls  and  the  term  flower  could  be  applied 
quite  as  well  to  the  association  of  these  organs  in  the  Pterido- 
phyta  and  gymnosperms. 

378 


DEVELOPMENT   OF   PLANTS 


379 


The  sporophylls,  like  all  organs  of  the  plant,  show  a  wide  range 
of  variation  and  they  are  generally  associated  with  more  or  less 
modified  leaf-like  organs  which  serve  to  protect  them.  These 
leaf-like  organs  are  known  as  the  floral  envelope  or  perianth  and 
doubtless  arose  in  many  forms  through  the  sterilization  and  modi- 
fication of  the  sporophylls  (Fig.  261  A,  i).  The  microsporo- 
phylls,  often  called  stamens,  usually  consist  of  a  stalk  or  fila- 
ment and  a  four-lobed  spore-bearing  part,  the  anther  (Fig.  261  A, 
2).  In  cross-section,  the  anther  is  seen  to  consist  of  four  spo- 


FIG.  261^4.  Flower  and  sporophylls  of  Angiosperms:  I,  flower  of  Sedum 
with  leaf-like  perianth,  p;  microsporophylls,  s;  megasporophylls,  c.  2, 
microsporophyll  of  the  buttercup,  showing  four-lobed  anther  and  filament. 

3,  diagram  of  a  cross-section  of  an  anther,  showing  the  breaking  down  of  the 
tissue  about  the  four  sporangia  and  the  beginning  of  the  opening  of  the  anther. 

4,  one  of  the  sporangia  from  a  young  anther,  as  seen  in  cross-section — m, 
spore  mother  cells.     The  large  cells  surrounding  the  mother  cells  are  nour- 
ishing cells,  known  as  the  tapetum,  and  disorganize  as  the  spores  mature. 
At  the  right  a  mother  cell  forming  four  microspores,  the  upper  one  being 
.characteristic  of  dicotyledons  and  the  lower  of  monocotyledons. 


380 


SPOROPHYLLS  OF  ANGIOSPERMS 


rangia  in  which  the  microspores  originate  in  fours  as  in  the 
preceding  groups  (Fig.  261  A,  3,  4).  At  maturity,  the  two 
sporangia  on  each  side  of  the  anther  usually  merge  into  one 
cavity,  owing  to  the  breaking  down  of  the  intervening  tissue. 
The  anther  opens  in  a  variety  of  ways,  as  by  slits  and  pores, 
permitting  the  scattering  of  the  spores  (Fig.  261  A,  3).  The 


FIG.  26i.B.  Megasporophyll  and  megasporangium :  I,  megasporophyll  of 
buttercup — s,  stigma;  st,  style;  o,  ovary.  2,  longitudinal  section  of  mega- 
sporophyll— mg,  megasporangium;  m,  megaspore,  the  mother  cell  having 
formed  four  spores  in  series,  but  the  end  one  only  develops;  *,  integument; 
mi,  micropyle. 

microsporophylls  are  often  externally  quite  suggestive  of  those 
noted  in  certain  of  the  Pinales,  but  the  megasporophylls  are 
essentially  different  from  any  other  group  considered,  in  that 
the  sporangia  are  inclosed  in  a  cavity  formed  in  the  sporophyll. 
This  peculiarity  is  one  of  the  important  characteristics  of  the 
angiosperms,  as  the  name  indicates.  The  megasporophyll,  often 
termed  the  pistil  or  carpel,  is  usually  a  rather  elongated,  flask- 
shaped  organ  with  a  hollow  swollen  base  and  consists  of  a  stigma, 
style  and  ovary  (Fig.  261.6,  i)  within  which  are  produced  the 
megasporangia  or  ovules.  Such  a  structure  would  result  if  the 
edges  of  the  leaf-like  megasporophylls  of  previous  groups  were 
inrolled  so  as  to  form  a  closed  organ.  The  megaspores  originate 


DEVELOPMENT  OF  PLANTS 


38i 


in  these  sporangia  as  already  noted  in  the  gymnosperms  and 
usually  but  one  megaspore  is  developed  in  each  sporangium  (Fig. 
26i£,  2). 

124.  Development  of  the  Flower  of  Angiosperms. — The  sporo- 
phylls  are  variously  associated  in  groups  that  are  commonly 
called  flowers.  In  its  simplest  form,  the  flower  may  be  defined 
as  a  minute  branch  or  receptacle  bearing  one  or  more  sporo- 
phylls.  Such  a  type  is  illustrated  in  the  cat- tail  (Fig.  262,  B,  C) 


FIG.  262.  Forms  of  primitive  flowers:  A,  inflorescence  of  Typha  or  cat- 
tail— mi,  region  bearing  only  flowers  with  microsporophylls;  mg,  flowers  with 
megasporophylls;  b,  bract.  B,  flower  consisting  of  two  microsporophylls 
which  are  sessile  on  a  short  stalk  that  has  numerous  hairs.  C,  flower  consist- 
ing of  one  megasporophyll — s,  stigma;  o,  ovary  surrounded  with  hairs.  D, 
early  appearance  of  the  inflorescence  of  Salix  or  willow.  £,  inflorescence 
bearing  only  megasporophylls.  F,  flower,  of  a  single  megasporophyll  with 
forked  stigma — 6,  bract;  n,  nectar  gland.  Gt  inflorescence  bearing  only 
microsporophylls.  H,  flower  of  two  microsporophylls. 

where  the  flower  consists  of  one  or  a  few  sporophylls  associated 
with  hairs,  and  also  in  the  willow  where  the  sporophylls  are 
developed  in  the  axil  of  a  minute  bract  (Fig.  262,  F,  H).  It 
should  be  noted  in  the  cat-tail  that  numerous  spirally  arranged 


382  PRIMITIVE   FLOWERS 

hairs  are  associated  with  the  sporophylls.  These  are  supposed 
to  represent  sterile  sporophylls  and  this  is  borne  out  by  the  fact 
that  primitive  flowers  are  characterized  by  just  such  an  arrange- 
ment of  their  sporophylls,  and  also  by  the  fact  that  in  an  allied 
genus  spirally  arranged  sporophylls  actually  occur.  These  simple 
types  of  flowers  are  often  developed  in  large  numbers  upon  an 
elongated  stem  (Fig.  262,  A,  D-G)  and  are  rather  suggestive 
of  a  strobilus  although  the  individual  flower  really  corresponds  to 
the  strobilus,  as  will  be  seen  especially  in  those  types  where 
the  sporophylls  are  numerous  and  arranged  spirally  upon  the 
receptacle  of  the  flower  (Fig.  264).  A  group  or  cluster  of  flowers 
is  called  an  inflorescence  in  contradistinction  to  the  solitary 
flower  developed  at  the  end  of  a  branch  or  stem.  It  is  notice- 
able in  these  primitive  angiosperms  that  the  micro-  and  mega- 
sporophylls  are  usually  borne  in  separate  flowers  or  inflorescences 
(compare  Finales)  which  are  developed  on  separate  plants  as 
in  the  willow  or  on  different  parts  of  the  same  plant  as  in  the 
cat-tail.  This  arrangement  is  probably  associated  with  the 
fact  that  the  advantages  of  crossing  or  the  transfer  of  the  micro- 
spores  of  one  flower  to  the  megasporophylls  of  another  is  effected 
by  the  wind.  In  higher  types,  which  include  the  great  majority 
of  angiosperms,  the  micro-  and  mega-sporophylls  are  developed 
in  the  same  flower  which  is  therefore  said  to  be  perfect  since  it 
contains  both  of  the  organs  essential  for  seed  production  (Fig. 
261  A,  i).  A  type  like  the  willow  is  termed  imperfect  because 
the  flower  lacks  one  kind  of  sporophyll.  Crossing  is  effected 
in  the  perfect  type  of  flower  by  the  earlier  ripening  of  the  micro- 
or  mega-sporophylls  and  often  also,  by  the  arrangement  of  the 
organs  of  the  flower  which  is  of  such  a  nature  that  the  microspores 
cannot  readily  reach  the  mega-sporophylls  of  the  same  flower. 
Insects  are  usually  the  agents  for  the  transport  of  the  micro- 
spores  in  such  cases.  Flowers  in  which  the  microspores  mature 
and  are  shed  from  the  anthers  before  the  stigmas  of  the  mega- 
sporophylls  are  ready  to  receive  them,  are  called  protandrous, 
meaning  that  the  microspores  which  develop  the  male  gametes, 
are  the  first  to  mature.  If  the  mega-sporophylls  become  recep- 
tive before  the  anthers  open,  the  flower  is  said  to  be  pro togy nous, 


DEVELOPMENT   OF   PLANTS 


383 


meaning  that  the  pistil  matures  first.  It  must  also  be  borne  in 
mind,  although  the  devices  for  effecting  a  crossing  are  almost 
universal  among  the  various  groups  of  angiosperms,  that  there 
are  equally  elaborate  provisions  for  the  transference  of  the 
microspores  of  perfect  flowers  to  the  stigmas  of  their  own  flower. 
This  is  called  autogamy  and  would  appear  to  be  a  provision  for 
setting  seed  in  case  crossing  fails. 

From  the  above  discussion  we  might  characterize  the  primitive 


FIG.  263.  Development  of  the  perianth:  A,  inflorescence  of  Quercus  or 
oak — mi,  inflorescence  with  flowers  bearing  only  microsporophylls;  mg, 
inflorescence  with  flowers  bearing  megasporophylls.  B,  flower  of  oak,  en- 
larged, consisting  of  several  microsporophylls  and  a  perianth  of  minute  scale- 
like  organs.  C,  flower  of  Erythronium  or  fawn  lily.  The  perianth  of  six  con- 
spicuous leaf-like  organs.  D,  flower  of  Melandryum  or  day  pink — ca,  calyx  of 
green  sepals;  c,  corolla  of  five  delicate  petals. 

flowers  as  consisting  usually  of  a  large  and  indefinite  number  of 
sporophylls,  spirally  arranged  upon  the  receptacle  which  is  suf- 
ficiently elongated  as  to  permit  the  separate  attachment  of  each 
organ.  Following  the  development  of  this  type  of  flower,  there 
appeared  as  the  next  advance,  minute  outgrowths  about  the 
sporophylls,  known  as  the  perianth.  In  its  simplest  form  this 


EVOLUTION   OF  THE   FLOWER 


consists  of  a  few  scales  as  in  the  sweet  flag,  oak,  etc.  (Fig.  263,  B), 
but  in  higher  forms,  the  perianth  appears  as  the  conspicuous 
leafy  portion  of  the  flower  as  in  the  lily  (Fig.  263,  C).  Finally 
flowers  appear  in  which  the  leaves  of  the  perianth  become  dif- 
ferentiated into  an  outer  calyx  composed  of  several  green  sepals 
and  a  corolla  of  larger,  more  delicate  and  often  brightly-colored 
leaves,  called  the  petals  (Fig.  263,  D).  We  have  now  reached  a 
point  where  the  flower  is  said  to  be  complete,  consisting  of  all 
the  organs  that  are  normally  associated  in  the  flower. 


FIG.  264.  FIG.  265. 

FIG.  264.  Flower  of  strawberry  with  elongated  receptacle  bearing  nu- 
merous spirally  arranged  sporophylls:  A,  open  flower.  B,  flower  in  section, 
showing  arrangement  of  parts  upon  receptacle,  r,  which  forms  a  shallow  cup 
at  base  bearing  the  perianth  and  microsporophylls.  C,  the  fruit  or  enlarged 
receptacle  bearing  the  minute  spirally-arranged  megasporophylls. 

FIG.  265.  Flowers  with  shortened  receptacles:  A,  flower  of  Pyrola  with 
calyx  and  corolla  arranged  in  whorls  or  cycles.  B,  section  of  flower,  showing 
all  the  organs  in  cycles.  C,  flower  of  geranium.  D,  flower  with  corolla 
removed  to  show  the  coherence  of  the  five  megasporophylls  that  results  from 
the  shortening  of  the  receptacle. 

As  stated  above  the  simpler  type  of  flower  is  characterized 
by  numerous  sporophylls,  spirally  arranged  and  separately  at- 
tached to  an  elongated  receptacle.  One  of  the  most  important 
variations  that  appeared  in  the  evolution  of  the  flower  is  asso- 
ciated with  the  shortening  of  this  receptacle.  This  is  brought 


DEVELOPMENT  OF  PLANTS 


385 


about  by  the  checking  of  the  apical  growth  of  the  receptacle  and 
is  often  associated  with  a  more  or  less  extended  growth  of  its 
basal  region.  These  changes  affected  the  flower  in  a  most  pro- 
found way.  A  crowding  resulted,  organs  were  reduced  in  number 
and  their  spiral  arrangement  upon  the  receptacle  became  so 
flattened  that  the  various  sets  of  organs  appear  to  arise  in  whorls 
or  cycles  (Fig.  265,  A,  B).  These  cyclic  flowers  are  characteristic 
of  all  the  higher  orders  of  angiosperms  and  no  one  character  is  of 


B 


D 


FIG.  266.  Forms  of  adhesion  that  result  from  shortening  of  receptacle: 
A,  flower  of  rose.  B,  section  of  flower,  showing  the  lower  portion  of  recep- 
tacle forming  a  cup  about  the  megasporophylls,  mg,  and  bearing  the  other 
organs  of  the  flower.  C,  inflorescence  of  comfrey,  Symphytum.  D,  flower 
enlarged  in  section  to  show  adhesion  of  microsporophylls,  mi,  to  the  tubular 
corolla. 

more  value  in  enabling  us  to  state  whether  a  plant  is  of  high  or 
low  rank.  As  the  cyclic  habit  became  established,  so  the  number 
of  organs  in  each  whorl  became  constant.  Thus  at  a  certain 
point  in  the  evolution  of  the  monocotyledons  you  will  find  the 
organs  usually  in  threes,  whereas  in  cyclic  dicotyledons  whorls 
with  four  or  five  organs  are  the  rule. 

The  crowding  of  the  organs  on  the  receptacle  also  caused  them 
to  develop  en  masse  (as  a  unit),  not  as  separate  organs.  This 


386  EVOLUTION   OF  THE   FLOWER 

tendency  is  especially  noticeable  in  the  megasporophylls,  which 
very  frequently  form  a  compound  megasporophyll  (Fig.  265, 
C,  D),  and  in  the  same  way  the  sepals  and  petals  may  appear  as 
a  more  or  less  tubular  calyx  and  corolla  (Fig.  266,  C).  The 
crowding  also  led  to  the  mass  growth  of  the  organs  of  adjacent 
sets  or  whorls.  This  is  often  seen  in  the  case  of  the  petals  and 
microsporophylls,  the  latter  organs  appearing  to  arise  from  the 
corolla  (Fig.  266,  D). 

The  receptacle  plays  an  important  role  in  all  cases  of  mass 
growth.  As  stated  above,  its  apical  portion  ceases  to  elongate 
at  an  early  period  while  the  basal  part  continues  active,  forming  a 
cup  about  the  ovaries.  The  sepals,  petals,  and  even  the  micro- 
sporophylls are  often  so  associated  with  this  basal  growth  of 
the  receptacle  that  they  arise  en  masse  about  the  ovaries.  This 
growth  may  be  so  slight  that  a  careful  examination  of  a  section 
of  a  flower  is  required  to  detect  it  or  a  conspicuous  cup-like  struc- 
ture may  be  formed,  as  in  the  rose  (Fig.  266,  A,  B).  This  type 
of  flower  is  termed  perigynous,  meaning  that  the  receptacle  and 
other  organs  form  a  more  or  less  conspicuous  cup  about  the 
ovaries.  In  simpler  types,  as  the  spiral  flowers,  it  can  be  seen 
that  the  megasporophylls  arise  at  the  top  of  the  receptacle  and 
that  each  of  the  other  organs  arises  at  a  point  just  below  the 
next  inner  one.  Such  flowers  are  called  for  this  reason  hypo- 
gynous,  meaning  below  the  ovaries. 

Very  frequently  the  basal  growth  of  the  receptacle  also  involves 
the  ovaries  which  becomes  distinct  from  the  mass  growth  at 
various  stages  in  their  development.  Consequently  the  other 
organs  of  the  flower  appear  to  arise  from  the  sides  or  from  the 
top  of  the  ovaries;  the  flowers  being  partially  or  completely 
epigynous,  meaning  that  the  organs  of  the  flower  are  developed 
upon  the  ovary  (Fig.  267,  B,  D).  Often  in  completely  epigynous 
flowers  the  receptacle  elongates  very  slightly  and  the  basal 
portion  grows  up  around  the  apex  forming  a  cup-like  cavity 
which  is  roofed  over  by  the  megasporophylls.  The  sporangia 
or  ovules  usually  arise  from  the  walls  of  the  cavity  thus  formed 
and  not  from  the  walls  of  the  megasporophylls  at  the  top  of  the 
cavity  (Fig.  267,  D).  The  calyx,  corolla  and  microsporophylls 


DEVELOPMENT  OF  PLANTS 


387 


arise  from  the  top  of  the  structure  thus  formed  and  these  organs 
may  be  developed  separately  or  there  may  be  varying  degrees  of 
mass  growth  as  noted  .in  the  perigynous  flower. 

Another  feature  to  be  noted  in  connection  with  the  evolution 
of  the  flower  is  its  symmetry.     In  the  lower  types,  the  organs 


FIG.  267.  FIG.  268. 

FIG.  267.  Adhesion  due  to  basal  growth  of  receptacle:  A,  flower  of  saxi- 
frage. B,  section  of  flower,  showing  megasporophyll  partially  inclosed  by 
receptacle.  C,  inflorescence  of  red  currant.  D,  section  of  flower,  showing 
the  receptacle  forming  a  sporangial  cavity  that  is  covered  at  the  top  by  the 
megasporophylls.  The  other  organs  arise  from  the  top  of  this  structure, 
the  microsporophylls  adhering  to  the  corolla. 

FIG.  268.  Irregular  or  zygomorphic  flower  of  honeysuckle:  ca,  calyx; 
c,  corolla  of  five  unequal  cohering  petals,  s,  stigma. 

of  a  set  are  alike  and  radially  arranged  about  the  center  of  the 
flower  (Fig.  267,  A).  This  is  the  regular  or  actinomorphic  type 
of  flower,  meaning  radially  symmetrical.  In  many  of  the  orders 
of  angiosperms  one  or  more  members  of  a  set  are  different  from 
the  others,  thus  destroying  the  radial  symmetry  (Fig.  268). 
This  is  the  irregular  or  zygomorphic  type  of  flower,  meaning 
yoke-form.  Such  flowers  can  be  cut  into  two  similar  halves  in 
but  one  plane. 

It  must  not  be  understood  that  evolution  of  the  flower  has 


388  GAMETOPHYTE  OF  ANGIOSPERMS 

progressed  steadily  through  the  various  changes  outlined  above 
and  that  consequently  in  the  following  lessons  we  can  begin  with 
the  most  primitive  type  and  proceed  by  regular  steps  to  the 
highest  forms.  The  various  orders  of  angiosperms  have  doubt- 
less been  derived  from  several  distinct  stocks  and  they  have 
not  only  varied  in  different  degrees  but  especially  will  it  be  noted 
that  some  orders  have  a  tendency  to  emphasize  certain  forms  of 
these  variations,  while  in  other  alliances,  the  variations  will 
proceed  along  quite  different  lines.  These  various  modifications 
have  been  retained  because  they  were  of  advantage  to  the  plant 
(p.  144).  The  cause  of  the  variations  is  unknown.  Similar 
lines  of  variations  also  appear  among  the  insects,  and  singularly 
these  modifications  of  the  flower  and  insect  have  been  mutually 
beneficial  the  one  to  the  other.  In  the  simpler  forms  of  flowers 
the  microspores  are  carried  by  the  wind,  as  in  the  gymnosperms. 
In  the  higher  types  odor  and  nectar  glands  appear  and  bright 
colors  which  serve  to  attract  insects.  This  type  of  flower  becomes 
modified  so  that  it  is  adapted  to  special  types  of  insects — all 
others  being  excluded.  In  this  way  the  microspores  are  carried 
with  greater  certainty  from  one  flower  to  another  of  the  same 
kind, 

125.  The  Gametophyte  of  the  Angiospermae. — The  angio- 
sperms have  developed  along  quite  distinct  lines,  but  they  show 
such  a  remarkable  uniformity  in  the  development  and  character 
of  the  gametophyte  generation  that  this  feature  of  their  life 
history  may  be  considered  at  this  point,  as  it  applies  to  all  forms. 
The  megasporangium,  also  called  the  .ovule,  originates  in  the 
cavity  of  the  ovary  at  various  points  known  as  the  placenta  (Fig. 
269,  A).  The  structure  of  the  sporangium  and  the  formation 
of  the  megaspore  are  very  similar  to  that  of  the  gymnosperms. 
More  often  two  integuments  are  formed  and  the  sporangium  or 
the  stalk  which  supports  it  becomes  curved  so  that  it  very  fre- 
quently is  turned  completely  over  (Fig.  269,  B).  The  germina- 
tion of  the  megaspore  results  in  a  female  gametophyte  which  is 
very  much  more  reduced  than  in  the  case  of  the  gymnosperms. 
As  the  megaspore  enlarges,  disorganizing  the  cells  of  the  sporan- 
gium, also  called  the  nucellus,  its  nucleus  divides  and  the  daugh- 


DEVELOPMENT  OF  PLANTS 


389 


ter  nuclei  move  to  each  end  of  the  spore  (Fig.  270,  A).  Each 
of  these  nuclei  divides  twice,  forming  four  nuclei  at  either  end 
of  the  spore  (Fig.  270,  B,  C).  This  generally  completes  the 
nuclear  divisions  in  the  gametophyte.  Only  a  few  cases  are 


FIG.  269.  Development  of  the  megasporangium  and  megaspore:  A,  sec- 
tional view  of  the  pepper-grass,  Lepidium.  This  flower  has  two  cohering 
sporophylls,  only  a  portion  of  the  right-hand  one  being  shown,  s,  stigma 
with  protruding  cells  to  receive  the  microspores;  mg,  megasporangium  con- 
taining a  single  spore  mother  cell,  me.  Two  integuments,  i,  are  growing  up 
about  the  sporangium.  B,  later  stage  of  development,  the  megasporangium, 
mg,  becoming  inverted  and  completely  covered  by  the  integument.  The 
mother  cell  of  A  has  formed  four  daughter  cells  in  series  and  not  in  tetrads 
as  in  the  Pteridophyta.  The  innermost  cell  of  the  series,  ms,  only  matures 
as  a  megaspore;  m,  micropyle;/,  stalk  or  funiculus  of  sporangium. 

known  where  a  larger  number  of  nuclei  are  formed — compare 
with  the  gymnosperms.  A  nucleus  from  each  of  these  groups, 
called  the  polar  nuclei,  now  approach  each  other  and  fuse, 
forming  a  single  large  nucleus  that  is  usually  called  the  endo- 
sperm nucleus.  This  growth  of  seven  cells  represents  the  female 
gametophyte.  The  three  outer  or  micropylar  cells  are  not  pro- 


390 


GAMETOPHYTE   OF  ANGIOSPERMS 


vided  with  walls  and  consist  of  a  rather  larger  cell,  the  female 
gamete,  and  two  nourishing  cells,  the  synergids  or  helpers  (Fig. 
271).  The  inner  group  or  antipodal  cells  usually  have  walls  and 
they  are  either  soon  disorganized  and  absorbed  by  the  enlarging 
gametophyte  or  they  may  remain  as  permanent  features  of  the 
gametophyte  for  a  long  time  and  even  increase  greatly  in  number, 
serving  to  nourish  the  gametophyte  by  absorbing  food  from  the 
sporangium.  The  endosperm  nucleus  plays  a  very  important 
role  in  the  development  of  the  sporophyte,  for  as  soon  as  fertiliza- 


B 


FIG.  270.     Germination  of  the  megaspore:  A,  first  division  of  the  mega- 
spore.     B,  second  division  of  the  nuclei.     C,  final  division  of  the  nuclei. 

tion  has  been  effected  it  forms  by  repeated  division  a  mass  of  cells 
that  completely  fills  the  entire  space  within  the  enlarging  spore, 
thus  providing  food  for  the  nourishment  of  the  sporophyte.  At 
first  sight  it  would  appear  impossible  to  compare  the  various  cells 
of  this  peculiar  gametophyte  with  the  tissues  of  the  female  gam- 
etophyte of  the  gymnosperms  or  ferns.  It  has  been  suggested 
that  the  female  gamete  and  the  synergids  are  the  remains  of  three 
archegonia,  only  one  of  which  is  usually  capable  of  being  fertilized, 
and  that  the  antipodal  cells  are  a  remnant  of  the  numerous  vege- 
tative cells  of  the  gametophyte.  The  peculiar  formation  of  the 
endosperm  nucleus  through  the  fusion  of  the  two  polar  cells  is 
looked  upon  as  a  nourishing  device  to  give  the  endosperm  nucleus 


DEVELOPMENT   OF   PLANTS  391 

the  power  to  grow  and  form  a  tissue  that  supplements  the  anti- 
podal cells  Wid  so  takes  the  place  of  the  nourishing  gametophyte 
of  the  gymnosperms.  According  to  this  view,  the  endosperm  is 
a  delayed  prothallial  growth  which  does  not  take  place  until 
fertilization  is  effected.  There  is  considerable  evidence  to  justify 


FIG.  271.  Section  of  a  megasporangium  of  lily,  showing  the  mature  female 
gametophyte:  9 ,  female  gamete,  below  which  are  two  synergids;  p,  the 
two  polar  nuclei  uniting  to  form  the  endosperm  nucleus;  a,  antipodal  cells; 
mi,  micropyle;  i,  integuments;  /,  funiculus  in  which  a  vascular  bundle,  v, 
has  been  formed  to  transport  foods  to  the  sporangium. 

this  conclusion.  In  the  Pteridophyta  the  archegonia  are  formed 
at  the  close  of  the  prothallial  development.  Among  the  species 
of  the  Gymnospermae  there  are  several  examples  indicating  that 
the  archegonia  are  formed  at  earlier  and  earlier  stages  in  the 
development  of  the  gametophyte  (p.  367).  Possibly  we  have  in 
the  Angiospermae  a  final  condition  where  the  homologues  of  the 


392 


GAMETOPHYTE  OF   ANGIOSPERMS 


archegonia  are  developed  at  the  very  start  of  the  gametophytic 
growth.  The  nicety  of  such  a  sequence  of  development  is  alto- 
gether admirable.  In  the  ferns,  if  fertilization  is  not  effected, 
the  prothallial  growth  is  wasted,  but  in  the  angiosperms  there  is 
no  prothallial  growth  without  fertilization. 

The  male  gametophyte  presents  several  features  suggestive  of 
the  gymnosperms.  The  germination  of  the  microspores  usually 
begins  within  the  sporophylls,  and  by  the  time  that  they  are  shed 
and  carried  to  the  stigma,  their  nuclei  have  already  divided  once 
and  each  spore  consists  of  a  large  tube  cell  and  an  antheridial  cell 
(Fig.  272,  B).  There  is  very  rarely  a  trace  of  the  vegetative 


FIG.  272.  Germination  of  the  microspore:  A,  mature  microspore  of  lily. 
5,  first  stage  of  germination — t,  tube  cell;  a,  antheridial  cell.  C,  final  divi- 
sion, in  this  case  effected  while  in  the  microsporangium — t,  tube  cell;  a,  an- 
theridial cell  forming  directly  two  male  gametes.  D,  diagram  showing  the 
formation  of  the  tube  which  grows  down  the  style  and  finally  reaches  the 
female  gametophyte.  The  two  male  gametes,  g,  are  shown  passing  down 
the  tube;  t,  tube  nucleus.  All  the  figures  in  sectional  view. 

cells  of  the  gametophyte,  as  in  the  cycads  and  pines,  since  the 
necessity  for  these  cells  no  longer  exists.  The  stigma  is  generally 
provided  with  minute  outgrowths  or  papillae  derived  from  the 
epidermal  cells  which  serve  to  hold  the  microspores  (Fig.  269,  s). 
These  cells  of  the  stigma  usually  secrete  a  sugary  solution  which 
nourishes  the  microspores  and  causes  a  continuation  of  their  ger- 
mination. It  is  noteworthy  that  these  microspores  may  be  made 
to  germinate  by  placing  them  in  sugar  solutions,  but  the  approxi- 


DEVELOPMENT  OF  PLANTS  393 

mate  strength  of  the  solution  on  the  stigma  must  be  determined 
in  order  to  prepare  a  solution  suitable  for  their  development. 
The  tube  cell,  stimulated  by  the  secretion  of  the  stigma,  ruptures 
the  outer  wall  of  the  spore,  which  is  provided  with  one  or  more 
thin  places  to  favor  this  growth,  and  protrudes  as  a  delicate  tube. 
This  tube,  owing  to  the  fact  that  it  is  repelled  by  the  oxygen  of 
the  air,  grows  down  into  the  tissues  of  the  style  which  are  really 
a  continuation  of  the  stigma.  These  tissues  of  the  style  are  usu- 
ally looser  and  provided  with  abundant  foods  which  are  deposited 
in  the  cells  just  ahead  of  the  elongating  tube  to  nourish  and  direct 
it  in  its  growth.  In  this  way  the  tube  is  directed  down  the  style 
to  the  cavity  of  the  ovary  where,  owing  to  the  attractive  influence 
of  the  organic  substances  in  the  sporangium,  possibly  in  the  syn- 
ergids,  it  usually  turns  out  into  the  cavity  of  the  ovary,  enters 
the  micropyle  and  works  its  way  through  the  sporangium,  and, 
unlike  the  Pinales, '  enters  the  female  gametophyte  generally 
alongside  of  one  of  the  synergids  (Fig.  273).  The  antheridial  cell 
usually  divides,  forming  directly  two  motionless  male  cells  during 
the  elongation  of  the  tube,  and  in  other  cases  these  two  cells  are 
already  formed  when  the  microspores  are  discharged  from  their 
sporangia.  In  none  of  these  cases  is  there  any  indication  in  the 
division  of  the  antheridial  cell  of  the  formation  of  a  wall  cell  as  in 
the  gymnosperms,  so  that  the  male  gametophyte  of  the  Spermato- 
phyta  presents  a  very  regular  series  of  reductions  from  the  cycads 
to  the  pines  and  thence  to  the  angiosperms,  where  it  consists  of  a 
tubular  growth  containing  three  naked  cells  (Fig.  272,  D).  This 
development  of  the  male  gametophyte  requires  from  a  day  to 
several  months  and  is  quite  independent,  apparently,  of  the  dis- 
tance that  it  has  to  traverse  in  reaching  the  female  gametophyte. 
The  male  cells  are  often  somewhat  elongated  and  even  spirally 
coiled  and  carried  to  the  end  of  the  tube,  as  in  the  Pinales. 

126.  Fertilization. — The  end  of  the  tube  finally  ruptures,  owing 
to  the  tension  of  the  fluids  that  gradually  accumulate  in  it,  and 
the  male  cells  are  forcibly  expelled  into  the  sac-like  cavity 
of  the  female  gametophyte  (Fig.  273).  One  of  the  male  cells 
passes  over  to  and  fuses  with  the  female  gamete,  thus  forming 
the  gametospore;  and  the  other  male  cell  unites  with  the  polar 
26 


394 


SPOROPHYTE   OF  ANGIOSPERMS 


nuclei,  thus  forming  the  endosperm  nucleus  through  a  triple 
fusion.  '  While  this  process  is  probably  in  the  nature  of  a  rein- 
forcement, enabling  the  endosperm  nucleus  to  perform  its  work, 
it  is  noteworthy  that  the  qualities  of  the  male  parent  are  trans- 


FIG. 273.  Section  of  the  micropylar  end  of  the  megasporangium,  show- 
ing the  process  of  fertilization.  The  tube,  t,  has  passed  through  the  micro- 
pyle,  entered  the  female  gametophyte  and  ruptured,  discharging  the  male 
cells.  One,  <?,  is  shown  fusing  with  the  female  gamete,  9,  and  the  other 
one,  cf',  is  uniting  with  the  two  polar  nuclei,  thus  making  a  triple  fusion  in 
the  formation  of  the  endosperm  nucleus;  s,  one  of  the  synergids;  i,  integu- 
ments. 

mitted  to  the  endosperm  cell  just  as  though  this  fusion  were  a 
sexual  process. 

127.  The  Germination  of  the  Gametospore. — After  fertiliza- 
tion the  endosperm  nucleus  divides  repeatedly,  and  usually  the 
resulting  nuclei  become  arranged  about  the  walls  of  the  sac-like 
gametophyte  which  may  now  be  called  the  embryo  sac  (Fig. 
274).  Later  the  endosperm  cells  develop  walls  and  by  further 
division  completely  fill  the  embryo  sac  with  cells.  This  mass  of 
cells  is  called  the  endosperm  and  the  method  of  its  development 
is  exactly  similar  to  that  of  the  female  gametophyte  of  the  gym- 


DEVELOPMENT   OF   PLANTS 


395 


nosperm,  and  it  serves  the  same  purpose,  namely,  to  nourish  the 
young  sporophyte  or  embryo.  Recall,  however,  that  it  originates 
in  a  different  manner  and  at  a  different  time  in  the  life  cycle. 
The  germination  of  the  gametospore  and  the  formation  of  the 
embryo  vary  so  greatly  that  only  a  very  general  statement  can 
be  made.  Following  fertilization,  the  gametospore  becomes  sur- 
rounded by  a  cell  wall  and  attached  to  the  wall  of  the  embryo 
sac  (Fig.  275,  A).  It  now  begins  to  elongate  and  its  nucleus 
divides  several  times,  forming  a  row  of  cells  (Fig.  275,  A,  B). 
This  is  the  proembryo,  the  terminal  cell  being  known  as  the 
embryo  cell  and  the  remaining  cells  as  the  suspensor.  Less 


FIG.  274.  Sectional  view  of  megasporophylls  of  Lepidium  shortly  after 
fertilization  (see  Fig.  269);  en,  early  development  of  endosperm  cells  about 
wall  of  embryo  sac;  p,  young  sporophyte  developing  from  gametospore; 
mi,  micropyle;  s,  stigma;/,  funiculus. 

commonly  the  proembryo  is  a  mass  of  cells  without  any  differ- 
entiation. This  appears  as  a  radical  departure  from  the  gymno- 
sperms  where  the  formation  of  free  nuclei  characterized  the 
germination  of  the  gametospore.  But  in  passing  from  the  cycads 
to  the  pines  there  is  a  steady  decline  in  the  number  of  free  cells 
formed  and  indeed  in  one  case  in  the  latter  group  the  proembryo 


396 


SPOROPHYTE   OF  ANGIOSPERMS 


consists  of  a  row  of  cells.  The  synergids,  which  are  partly  con- 
sumed during  the  entrance  of  the  tube  cells  and  the  process  of 
fertilization,  usually  become  entirely  disorganized  and  absorbed 
during  these  early  stages  of  germination.  The  embryo  is  formed 
by  the  repeated  divisions  of  the  embryo  cell  of  the  pro-embryo, 


B  A 

FIG.  275.  Stages  in  the  germination  of  the  gametospore  of  Lepidium, 
sectional  view:  A,  micropylar  end  of  embryo  sac,  showing  the  enlarging 
gametospore  provided  with  cell  wall  and  attached  to  wall  of  sac.  B,  later 
growth — s,  suspensor;  e,  embryo  cell;  en,  endosperm  cells.  C,  pro-embryo 
after  first  division  of  embryo  cell.  D,  further  divisions  of  embryo  cell,  show- 
ing formation  of  an  epidermis  and  a  central  stem  region.  E,  later  growth, 
two  growing  regions,  the  cotyledons,  appearing  on  the  sides  of  the  stem.  F, 
micropylar  end  of  the  embryo  sac  in  which  the  embryo  cell  has  formed  a  small 
plant  or  embryo,  consisting  of  two  cotyledons,  c;  stem,  st,  which  terminates 
in  the  root,  r;  the  endosperm  cells,  en,  are  being  absorbed  by  the  enlarging 
embryo;  s,  suspensor. 

assisted  to  a  varying  extent  by  one  or  more  of  the  adjoining  cells 
of  the  suspensor  (Fig.  275,  C-F).  The  remaining  cells  of  the 
suspensor  are  ultimately  disorganized  or  they  may  increase 
greatly  in  size  and  themselves  become  the  principal  means  of 


DEVELOPMENT   OF   PLANTS  397 

absorbing  food  and  transferring  it  to  the  embryo.  The  structure 
of  the  mature  embryo  varies  greatly.  In  some  genera,  as  in  the 
orchids,  Indian  pipe,  etc.,  it  remains  rudimentary,  consisting  of 
only  a  few  cells.  Among  the  monocotyledons,  the  embryo  cell 
frequently  produces  the  single  cotyledon,  while  the  next  under- 
lying cell  of  the  suspensor  forms  the  root  and  the  laterally-placed 
growing  point  of  the  stem  (Fig.  279).  The  pro-embryo  of  dico- 
tyledons is  frequently  a  filament  of  cells  of  varying  length  and 
the  embryo  cell,  by  a  regular  series  of  divisions,  gives  rise  to  the 
stem,  two  laterally-placed  cotyledons  and  all  of  the  root,  save  the 
tip,  which  is  formed  from  the  cells  adjoining  the  embryo  cell 
(Fig.  276,  A). 

128.  The  Fruit  and  Seed. — Various  changes  occur  in  the 
sporangium  during  the  growth  of  the  embryo.  More  frequently, 
perhaps,  the  embryo  sac  enlarges,  absorbing  all  the  cells  within 
the  integument,  and  it  becomes  filled  with  endosperm  cells.  In 
a  case  like  this,  the  embryo  either  remains  small  and  embedded 
in  the  endosperm  (Fig.  279),  or  the  embryo  may  entirely  con- 
sume the  endosperm,  the  food  in  this  case  being  stored  in  the 
cotyledons  (Fig.  276,  A).  Less  commonly,  the  embryo  sac  ab- 
sorbs only  a  portion  of  the  sporangial  tissue  (often  called  the 
perisperm)  and  consequently  the  embryo  is  associated  with  a 
varying  amount  of  endosperm,  which  in  turn  is  surrounded  with 
sporangial  cells  or  perisperm,  as  in  the  water  lily  (Fig.  276,  B). 
The  integument  usually  undergoes  pronounced  changes  during 
this  growth,  becoming  hard  and  tough  to  protect  the  parts  within, 
and  often  developing  appendages  of  various  kinds,  as  hairs  and 
wings,  to  promote  the  distribution  of  the  seed  (see  gymnosperms, 
page  374). 

The  stimulus  of  fertilization  extends  beyond  the  changes 
wrought  in  the  sporangium.  This  is  particularly  noticeable  in 
the  megasporophyll  and  often  in  adjacent  parts  which  keep  pace 
with  the  growth  of  the  sporangium  and  often  undergo  remarkable 
transformations.  The  result  of  this  total  growth  is  called  the 
fruit,  while  the  term  seed  is  restricted  to  the  modified  sporangium 
with  its  integument  and  embryo.  The  megasporophyll  may  form 
a  firm  coat  that  is  closely  attached  to  the  seed,  as  in  the  corn  and 


398 


SEED   AND   FRUIT   OF   ANGIOSPERMS 


other  grasses.  Such  a  fruit  is  called  a  grain.  Again  the  tough 
walls  of  the  megasporophyll  are  free  from  the  seed,  as  in  the 
buttercup,  forming  a  fruit  known  as  the  akene.  The  sporophylls 
may  become  papery  or  hard  and  split  open  to  scatter  the  seed. 
Where  a  single  megasporophyll  behaves  in  this  way  and  opens  by 
two  valves,  the  fruit  is  called  a  pod  or  legume,  example  the  bean, 


FIG.  276.  Seed  structure:  A,  section  of  a  nearly  mature  seed  of  Lepid- 
ium.  The  stem  of  the  embryo  is  differentiated  below  into  a  hypocotyl,  hy, 
and  above  into  an  epicotyl,  pi,  commonly  known  in  the  seed  as  the  plumule, 
r,  root  with  root  cap;  c,  the  two  cotyledons,  which  are  bent  over,  lying  one 
upon  the  other;  v,  vascular  bundles  extending  through  the  stem  into  the 
cotyledons,  where  they  form  a  network  of  veins;  en,  remains  of  endosperm. 
B,  section  of  seed  of  water  lily  (after  Conard) — e,  embryo,  surrounded  by  a 
layer  of  endosperm  cells;  mg,  cells  of  the  megasporangium ;  *,  integument. 

or  if  by  one  valve  the  fruit  is  termed  a  follicle,  example  the 
peony.  Frequently  wing-like  processes  develop  from  the  mega- 
sporophyll, as  in  the  maple  and  ailanthus,  which  are  of  service  in 
distribution  and  in  other  ways,  as  in  the  manufacture  of  foods, 
these  organs  often  being  green  during  the  development  of  the 
embryo.  Such  fruits  are  known  as  key  fruits  or  samaras.  In 
other  cases  the  sporophyll  becomes  fleshy,  forming  a  berry  as  in 
the  currant  or  the  inner  layer  forms' a  pit  or  stone,  while  the  outer 


DEVELOPMENT   OF   PLANTS  399 

layer  forms  the  pulp  and  skin,  as  in  the  cherry  and  peach,  fruits 
known  as  drupes.  In  many  fruits  the  receptacle  becomes  fleshy 
and  forms  the  major  portion  of  the  fruit,  as  in  the  fig,  apple, 
strawberry,  etc.,  and  in  the  pineapple,  the  axis  of  the  inflores- 
cence, as  well  as  parts  of  the  flower,  become  fleshy.  These  vari- 
ous devices  serve  primarily  to  preserve  the  life  of  the  embryo 
and  some  of  them  also  assist  in  distributing  the  seed. 

When  conditions  are  favorable  for  growth,  which  may  not  be 
till  several  years  after  the  seeds  have  been  scattered,  the  embryo 
renews  its  growth.  The  region  below  the  cotyledons,  the  hypo- 
cotyl,  or  base  of  the  cotyledon  itself,  elongates  and  pushes  out 
the  root  which  grows  down  into  the  soil,  and  by  a  later  growth 
the  stem  tip,  with  or  without  the  cotyledons  which  may  remain 
in  the  seed,  elongates  and  develops  the  characteristic  stem, 
branches,  leaves  and  sporophylls  of  the  parent  plant.  The  struc- 
ture and  variations  of  the  mature  sporophyte  have  been  presented 
in  the  first  part  of  the  work  (pages  7-145),  and  will  be  further 
considered  in  the  following  studies.  The  terms  sporangium, 
microsporophyll  and  megasporophyll  have  been  retained  up  to 
this  point  in  the  work  in  order  to  keep  clearly  before  you  the 
progress  and  relationship  of  the  variations  that  have  attended  the 
evolution  of  plant  life.  Now  that  we  have  arrived  at  the  highest 
and  largest  group  of  plants,  where  the  sporophylls  have  become 
highly  modified  and  curiously  associated  in  clusters  called  flowers, 
more  familiar  names  will  be  used  in  discussing  them,  i.  e.,  ovule 
for  the  megasporangium  and  its  integument,  stamen  for  micro- 
sporophyll and  pistil  for  megasporophyll.  The  terms  "carpel" 
and  "pistil"  are  synonymous  when  these  organs  are  distinct — 
i.  e.,  free  from  one  another;  but  when  they  cohere  to  form  a 
compound  pistil  the  term  "carpel"  is  employed  to  indicate  the 
number  of  pistils  that  are  united,  as  in  the  willow  the  pistil  is 
composed  of  two  carpels,  in  the  lily  of  three  carpels,  etc. 

While  the  sexual  generation,  mode  of  reproduction,  and  the 
character  of  the  tissues  are  very  similar  in  all  angiosperms,  two 
very  distinct  lines  of  variation  or  classes  are  to  be  noted:  A, 
the  Monocotyledones,  represented  by  the  grasses,  lilies,  and  or- 
chids; B,  the  Dicotyledones,  represented  by  our  common  trees, 
roses,  mints  and  daisies. 


400  MONOCOTYLEDONES 

Class  A.     Monocotyledones 

129.  General  Characteristics. — This  group  of  angiosperms  con- 
tains about  25,000  species  that  constitute  a  very  natural  alliance, 
owing  to  the  uniformity  and  simplicity  of  their  structures.  These 
features  may  be  due  to  the  uniform  conditions  under  which  these 
plants  live.  The  majority  of  Monocotyledones  are  moisture- 
loving  plants  and  are  therefore  exposed  to  very  constant  condi- 
tions, which  would  naturally  result  in  less  stimulation  and  conse- 
quent variation  than  in  the  case  of  plants  exposed  to  the  varying 
conditions  of  drier  soils.  Some,  to  be  sure,  have  become  adapted 
to  dry  and  even  arid  regions,  owing  to  peculiar  modifications  of 
their  leaves  or  stems,  as  in  the  grasses,  certain  bulbous  plants,  etc. 

The  leaves  are  smooth  and  simple,  lance-shaped  or  linear  in 
outline,  sessile  and  often  attached  to  the  stem  by  sheathing  bases. 
The  veins  do  not  end  in  free  branches  on  the  margins  of  the 
leaves,  and  as  a  result  of  this  closed  venation  the  leaves  are  usu- 
ally entire  and  destitute  of  teeth  or  lobed  margins.  In  many 
instances  the  principal  veins  are  quite  parallel,  but  whatever  the 
arrangement,  the  prominent  veins  are  connected  by  very  minute 
veinlets  that  form  an  inconspicuous  network  or  reticulation 
throughout  all  parts  of  the  leaf  (Fig.  277).  The  stems  are  com- 
posed largely  of  parenchyma,  through  which  are  scattered  numer- 
ous vascular  bundles  as  in  some  ferns  (Fig.  278) — see  page  324. 
These  bundles  rarely  develop  a  cambium  (Fig.  60)  and  conse- 
quently the  stem  does  not  increase  materially  in  diameter,  and 
usually  it  is  columnar  in  appearance.  In  many  instances  the 
stems  are  reduced  in  size  and  are  subterranean,  the  bulb  and 
rhizome  being  common  forms  of  stems  which  send  up  annually 
short-lived  aerial  branches.  As  a  rule  the  stems  do  not  branch, 
owing  to  the  failure  of  the  buds  in  the  axils  of  the  leaves  to 
develop. 

The  flowers  are  also  of  rather  simple  and  uniform  structure, 
in  the  simplest  cases  being  imperfect  and  consisting  of  either 
spirally  arranged  stamens  or  pistils  without  perianth.  In  the 
higher  types  spiral  flowers  with  perianth  and  both  kinds  of  sporo- 
phylls  appear,  and  these  give  place  to  forms  in  which  the  organs 
are  arranged  in  successive  whorls  of  three  parts  each,  the  latter 


DEVELOPMENT  OF  PLANTS 


401 


type  of  flower  being  especially  common.  Perigynous,  epigynous 
and  irregular  flowers  are  of  less  common  occurrence.  The  most 
conspicuous  feature  of  the  monocotyledons  is  the  embryo,  which 
has  a  single  terminal  cotyledon  and  a  laterally  developed  stem  tip 


B 


FIG.  277.  FIG.  278. 

FIG.  277.  Leaf  of  Solomon's  seal  with  closed  venation,  entire  margins, 
and  sessile  upon  the  stem,  i.  e.,  without  petiole. 

FIG.  278.     Cross-section  of  stem  of  rush:  v,  vascular  bundles;  st,  stereome. 

(Fig.  279).  Generally  the  root  and  stem  tip  are  set  free  from 
the  seed  by  the  elongation  of  the  basal  portion  of  the  cotyledon, 
while  the  cotyledon  remains  in  contact  with  the  endosperm  as  a 
food-absorbing  organ,  being  often  highly  modified  to  perform 
this  work.  An  examination  of  a  few  of  the  more  important 
orders  will  give  a  better  idea  of  the  characteristics  and  variations 
of  the  monocotyledons. 

130.  Pandanales,  the  Cat-tail  Order. — The  cat-tail,  Typha, 
may  be  taken  as  an  illustration  of  this  order  (Fig.  280).  These 
plants  live  in  wet  and  marshy  places  and  present  several  varia- 
tions that  adapt  them  to  such  conditions.  The  main  stem  is  a 
rhizome  that  branches  through  the  mud  and  sends  up  each  spring 
leafy  stems.  The  rhizome  grows  on  from  year  to  year,  and 
owing  to  the  decay  of  the  older  portions,  the  branches  become 
independent  plants.  In  this  way  a  single  plant  may  spread  and 


402 


THE   PANDALES 


thickly  populate  an  extensive  marsh.  The  aerial  stems  live  but 
for  a  year  and  are  rather  weak,  but  owing  to  the  sheathing  bases 
of  the  leaves  they  become  sufficiently  rigid  to  support  a  heavy 
foliage  and  withstand  the  winds.  Notice  also  the  extreme  light- 
ness of  these  organs.  This  is  due  to  the  larger  air  spaces  which 
also  permit  a  ready  interchange  of  gases  from  the  leaves  to  all 
parts  of  the  plant,  even  throughout  the  submerged  rhizomes. 
You  would  naturally  expect  to  find  this  structure  in  all  aquatics 
since  the  roots  at  least  are  submerged  and  all  living  cells  require 
an  interchange  of  gases  (see  page  54).  The  leaves  are  long 


mT 

FIG.  279.  Sectional  view  of  seed  of  Veltheimia.  The  embryo  consisting 
of  a  large  cotyledon,  c,  and  laterally  placed  stem,  s,  below  which  is  the  root, 
e,  endosperm;  mi,  micropyle;/,  funiculus. 

and  narrow  and  point  nearly  straight  up  in  the  air.  This  pre- 
vents shading  and  permits  the  association  of  the  plants  in  dense 
colonies.  The  leaves  are  covered  with  a  waxy  coating  or  bloom 
to  prevent  the  adhesion  of  water  and  the  plugging  of  the  stomata. 
This  device  is  often  to  be  seen  on  the  leaves  of  plants  that  are 
subject  to  heavy  dews  or  rains.  Moisture  is  frequently  not 
evaporated  from  the  leaves  until  near  midday,  and  if  the  stomata 
become  filled  with  water,  there  can  be  no  interchange  of  gases 
for  photosynthesis  during  this  time. 

The  flowers  are  of  a  very  primitive  type,  consisting  of  naked 


DEVELOPMENT  OF   PLANTS 


403 


sporophylls  arranged  in  a  compact  inflorescence  that  assumes  a 
spike-like  structure  at  the  tip  of  the  stem.  The  sporophylls  are 
protected  until  mature  by  modified  sheathing  leaves  (Fig.  280,  4). 
Each  flower  in  the  upper  portion  of  the  spike  contains  only  two 
or  three  stamens  supported  upon  a  short  stalk,  which  is  asso- 
ciated with  hair-like  outgrowths  (Fig.  280,  B),  while  the  lower 
flowers  of  the  spike  bear  a  single  pistil  each  (Fig.  280,  C),  which 


mi 


FIG.  280.  FIG.  281. 

FIG.  280.  Inflorescence  and  fruit  of  Typha:  A,  inflorescence — b,  pro- 
tecting bracts  curving  away  from  the  sporophylls;  mi,  region  of  stamina te 
flowers;  mgt  region  of  pistillate  flowers.  B,  staminate  flower  consisting  of 
two  stamens  sessile  upon  a  short  stem.  C,  pistillate  flower  of  one  carpel — 
s,  stigma;  o,  ovary  containing  a  single  ovule.  D,  appearance  of  A  in  the 
fall — mi,  region  occupied  by  staminate  flowers;  mg,  the  pistils  have  increased 
greatly  in  size  during  the  ripening  of  the  seed. 

FIG.  281.  The  mature  fruit  of  Typha:  s,  remains  of  the  stigma;  o,  ovary; 
p,  elongated  pedicel  bearing  numerous  hairs.  Compare  Fig.  280,  C. 

consists  of  a  large  flat  stigma,  style  and  ovary  containing  a  single 
ovule  and  supported  upon  a  hairy  stalk  or  pedicel.     Note  the 


404  THE   PANDANALES 

association  of  numerous  hairs  with  both  the  stamens  and  pistils. 
These  organs  are  supposed  to  be  sterile  sporophylls  so  that  we 
have  here  a  very  good  illustration  of  a  primitive  type  of  flower. 
The  microspores  are  carried  to  the  stigmas  of  the  pistils  by  the 
wind. 

Such  wind  pollinated  flowers  are  characterized  by  several  fea- 
tures well  illustrated  in  the  cat-tail.  The  microspores  must  be 
produced  in  large  numbers  since  the  chance  of  one  reaching  the 
stigma  of  another  plant  rapidly  decreases  as  the  distance  tra- 
versed by  the  spores  increases.  This  probably  accounts  for  the 
growth  of  such  plants  in  dense  colonies  since  the  close  proximity 
of  the  plants  greatly  increases  the  chance  of  the  microspores 
reaching  the  stigmas.  The  Finales,  grasses,  willows,  oaks,  etc., 
are  other  illustrations  of  a  very  large  series  of  plants,  some 
10,000  in  number,  that  have  a  similar  habit.  Wind  pollinated 
flowers  are  inconspicuous  and  simple  in  structure.  You  will 
notice  that  showy  perianths,  nectar  and  perfume  glands  are 
developed  only  in  such  flowers  as  utilize  insects  for  the  distri- 
bution of  the  microspores.  The  stigmas  of  these  flowers  are 
usually  large  and  hairy  or  brush-like  and  conspicuously  exposed 
so  as  to  increase  the  chances  of  catching  the  microspores,  thus 
reducing  the  dangers  of  this  rather  risky  method  of  crossing. 
You  will  also  observe  that  wind  pollinated  flowers  are  usually 
characterized  by  having  imperfect  flowers,  the  stamens  and  pistils 
being  developed  on  different  parts  of  the  plant,  or  on  different 
plants.  By  this  arrangement,  the  advantages  of  crossing  are 
secured  and  it  also  happens  that  the  microspores  are  either 
scattered  before  the  stigmas  of  neighboring  flowers  are  mature, 
or,  more  frequently  after  they  have  withered  and  are  therefore 
no  longer  capable  of  catching  and  nourishing  the  spores.  In 
this  way  it  comes  about  that  the  microspores,  even  when  close 
to  the  pistillate  flowers  as  in  Typha,  are  often  only  of  service 
when  carried  to  some  earlier  flowering  plant  whose  stigmas  are 
mature.  In  the  cat- tail,  the  microspores  are  shed  a  day  or  so 
before  the  stigmas  on  the  same  spike  are  mature  and  so  there 
must  result  the  benefit  that  comes  from  crossing  two  more  or 
less  widely  separated  plants  (page  144).  The  stamens  soon 


DEVELOPMENT  OF  PLANTS  405 

perish  after  the  discharge  of  their  spores,  but  the  pistils  increase 
greatly  in  size  during  the  ripening  of  the  seed  and  form  con- 
spicuous brown  cylinders  (Fig.  280,  D).  As  winter  approaches, 
the  pedicel  below  the  ovary  and  the  hairs  attached  to  it  become 
greatly  elongated  and  the  sporophyll  is  thus  transformed  into 
a  very  stable  parachute  that  is  easily  detached  from  the  spike 
and  capable  of  floating  the  seed  in  even  the  lightest  winds  (Fig. 
281).  If  this  fruit  chances  to  fall  in  a  marsh,  the  torpedo-like 
seed  after  a  time  falls  from  the  ruptured  ovary  and  sinks  in  the 
water.  When  the  growth  of  the  seed  is  renewed,  the  base  of 
the  cotyledon  elongates,  pushes  off  the  lid  at  the  end  of  the  seed 
and  curves  down  so  as  to  bring  the  root  of  the  embryo  in  con- 
tact with  the  mud.  Hairs  now  develop  from  the  lower  part  of 
the  cotyledon,  anchoring  the  young  plant  to  the  ground  while 
the  tip  of  the  cotyledon  remains  in  the  seed  until  the  food  is 
absorbed,  when  it  is  withdrawn.  In  the  meantime,  roots  have 
developed  and  penetrated  the  soil  and  the  stem  tip  begins  to 
elongate,  lifting  up  the  cotyledon  and  making  possible  the  for- 
mation of  other  leaves.  The  curious  screw-pines,  Pandanus,  a 
large  group  from  the  oriental  tropics  and  frequently  seen  in  con- 
servatories, belong  to  this  order,  also  the  bur  reed  (Sparganium) , 
commbn  about  marshes  and  water  ways,  which  is  the  highest 
member  of  the  order. 

131.  Graminales,  the  Grass  and  Sedge  Order. — This  is  one  of 
the  largest  groups  of  the  Spermatophyta,  some  7,000  species 
being  known,  and  in  number  of  individuals  it  exceeds  all  others 
(Fig.  282).  These  plants  are  almost  universally  distributed 
over  the  earth  and  have  become  adapted  to  almost  every  condi- 
tion of  climate  and  soil.  The  ability  of  these  plants,  which  are  of 
aquatic  origin,  to  establish  themselves  upon  the  drier  and  more 
diversified  land  surface  doubtless  promoted  variations  and  ac- 
counts in  part  for  the  enormous  display  of  forms.  The  Grami- 
nales and  the  palms  (Principales)  which  belong  to  an  order 
related  to  Typha  are  about  the  only  groups  of  monocotyledons 
that  are  associated  in  sufficient  numbers  to  form  striking  features 
of  the  vegetation  of  a  country.  This  is  particularly  true  of  the 
Graminales  in  all  lands,  where  they  constitute  the  principal  vege- 


406 


THE  GRAMINALES 


tation  of  hills,  plains,  meadows  and  marshes.  These  plants  are 
also  of  the  greatest  economic  importance,  furnishing  a  variety  of 
foods,  as  wheat,  rye,  barley,  oats,  rice,  corn,  hay,  etc.  The 
bamboo  also  belongs  to  the  order.  The  stem  in  the  majority 
of  forms  is  a  rhizome  (as  in  Typha),  which  branches  extensively 
through  the  soil  and  sends  up  numerous  aerial  branches  (Fig.  282, 
E).  These  interwoven  rhizomes,  with  their  numerous  roots, 
form  the  firm  swards  of  meadows  and  prairies.  The  aerial  stems 
are  models  of  mechanical  construction.  The  ability  of  the  long 


FIG.  282.  Habit  of  growth  of  one  of  the  grasses:  A,  aerial  stem  termi- 
nating in  a  branched  inflorescence.  B,  underground  stem  or  rhizome  send- 
ing up  a  new  shoot  from  one  of  the  nodes.  C,  section  through  the  node  of 
a  stem  that  has  been  placed  horizontal,  showing  the  sheathing  leaf  base,  /, 
and  the  beginning  of  the  upward  curvature  of  the  stem. 

hollow  stems  of  the  grasses  to  support*  the  heavy  head  of  grain 
must  appeal  to  every  one.  An  examination  of  Fig.  282  will 
show  you  how  the  stem  is  reinforced  by  the  long  sheathing  base 
of  the  leaves  while  tough,  elastic  strands  of  stereome  fibers  are 
so  distributed  as  to  give  the  best  mechanical  support.  An  un- 


DEVELOPMENT   OF   PLANTS 


407 


usual  localization  of  the  growing  regions  also  characterizes  the 
grasses  which  enables  them  to  lift  up  again  their  stems  if  they 
by  any  means  become  prostrate.  In  case  the  stem  becomes  pros- 
trate, the  stimulus  of  gravity  causes  a  renewal  of  growth  in  the 
cells  at  the  base  of  the  node  on  the  side  of  the  stem  next  to  the 
ground,  thus  causing  the  stem  to  curve  up  (Fig.  282,  C).  You 
can  easily  demonstrate  this  peculiar  localization  of  growth  by 
cutting  off  a  growing  stem  of  grass  and  placing  it  horizontally 
with  one  end  embedded  in  moist  sand  and  noting  the  curvature 


FIG.  283.  Inflorescence  of  a  grass:  I,  tip  of  stem  with  flowers  arranged 
in  spike-like  inflorescences.  2,  a  single  spike  enlarged  at  time  of  flowering. 
4,  diagram  showing  structure  of  spike.  At  base  two  sterile  bracts,  above 
three  flowers,  each  enclosed  by  an  outer  firm  bract  and  an  inner  more  delicate 
bract.  /,  lodicules.  At  apex  of  spike  a  sterile  flower.  5,  another  species 
of  grass,  showing  the  scattering  of  the  microspores. 

of  the  node  after  a  day.  The  leaves  show  wide  variation  in 
structure  and  are  doubtless  one  of  the  factors  that  have  enabled 
these  plants  to  live  under  a  variety  of  conditions.  For  example, 
where  the  plants  are  exposed  to  drought  or  drying  winds  the 


408  THE   GRAMINALES 

epidermal  cells  are  very  much  thickened,  stomata  often  sunken 
in  narrow  furrows  and  practically  all  species  have  the  power  of 
rolling  up  the  leaves  in  dry  seasons  and  thus  reducing  the  leaf 
surface  and  lessening  transpiration. 

The  flowers  are  exceptionally  alike  in  structure  and  show  about 
the  same  state  of  floral  development.  While  resembling  those 
of  Typha  in  some  particulars,  they  present  several  features 
that  indicate  a  decided  advance  over  the  previous  group.  Fig. 
283  illustrates  the  common  types  of  inflorescence  found  among 
the  grasses.  The  flowers  are  arranged  on  elongated  branches, 
but  instead  of  a  large  bract  ensheathing  the  inflorescence  (see 
Typha),  each  flower  is  inclosed  by  one  or  more  small  bracts 
so  arranged  as  to  form  a  spike-like  structure  of  overlapping 
bracts  (Fig.  283,  2).  The  two  kinds  of  sporophylls  may  be 
arranged  in  separate  spikes  or  on  different  parts  of  the  same 
spike,  or,  as  in  many  grasses,  the  stamens  and  pistils  are  developed 
in  the  same  flower.  One  or  more  of  the  lower  bracts  of  a  spike 
are  usually  without  flowers  and  above  them  are  one  to  several 
flowers  enclosed  by  secondary  bracts  (Fig.  283,  4).  Removing 
the  secondary  bracts,  the  flower  proper  is  seen  (Fig.  284,  3^4, 
3jB).  This  consists  in  many  of  the  grasses  of  three  stamens 
and  one  pistil  and  usually  two  small  scale-like  organs,  the  lodi- 
cules,  which  assist  by  their  expansion  in  forcing  open  the  pro- 
tecting bracts  at  the  time  of  flowering.  The  stamen  consists 
of  a  long  filament  attached  to  the  middle  of  the  anther  so  that 
the  latter  organ  can  swing  at  the  end  of  the  filament,  an  arrange- 
ment known  as  the  versatile  anther.  The  stigma  is  often 
brightly  colored  and  of  a  delicate  feathery  character,  indicating 
that  the  plants  are  wind  pollinated.  The  ovary  contains  a 
single  ovule.  The  manner  of  flowering  of  the  numerous  genera 
of  this  order  varies.  In  many  cases  the  stigmas  are  first  ex- 
truded from  the  bracts  and  can  therefore  be  crossed  only  with 
the  microspores  from  some  earlier  flowering  plant.  In  other 
instances,  stigmas  and  anthers  are  extended  together  and  in 
some  genera  no  injury  results  from  the  transfer  of  the  micro- 
spores  to  the  stigmas  of  the  same  flower.  It  is  worth  any  one's 
time  to  note  the  time  of  flowering,  the  region  on  the  spike  where 


DEVELOPMENT   OF   PLANTS 


409 


it  begins  and  the  manner  of  opening  of  the  bracts  and  extension 
of  anther  and  stigma.  Many  open  between  6  and  7  A.  M.  on 
pleasant  days.  Owing  to  the  swelling  of  the  lodicules,  or  of 
the  base  of  the  bracts,  these  latter  organs  are  forced  apart,  per- 


FIG.  284.  FIG.  285. 

FIG.  284.  Flower  and  fruit  of  grass:  $A,  a  single  flower  with  the  two 
enveloping  bracts  opened,  exposing  the  stamens  and  pistil  with  feathery 
stigmas.  3.8,  flower  with  outer  firm  bract  removed — /,  lodicules;  st,  stigma. 
6,  mature  fruit  or  grain — 0,  region  of  embryo.  7,  section  through  base  of 
grain,  showing  the  root,  stem  leaves,  and  scutellum,  sc,  or  absorbing  organ 
of  the  embryo;  en,  endosperm.  jA,  diagram  of  a  few  of  the  outer  cells  of 
the  scutellum,  sc. 

FIG.  285.  Inflorescence  of  one  of  the  sedges,  Carex:  p,  spike  of  pistillate 
flowers,  each  pistil  is  surrounded  by  a  papery  sac,  through  which  the  style 
and  stigma  protrude;  s,  spike  bearing  staminate  flowers.  Note  the  triangu- 
lar stem,  a  characteristic  of  this  large  genus  of  over  1,000  species. 

mitting  the  extension  of  the  stigma  and  anthers.  The  filament 
quickly  elongates  and  soon  curves  so  as  to  allow  the  anther  to 
swing  back  and  forth  in  the  wind.  The  two  lobes  of  the  anther 
now  curve  apart  and  open  at  their  ends  by  a  narrow  slit,  forming 
27 


410  THE  ARALES 

a  cup  in  which  the  microspores  collect  (Fig.  283,  5).  As  the 
rocking  of  the  anther  in  the  wind  sifts  out  the  spores,  more 
rattle  down  into  the  cup  and  so  a  very  gradual  scattering  of  the 
spores  is  effected.  This  entire  process  may  be  effected  in  a  few 
minutes,  and  in  the  case  of  the  wheat  each  flower  is  said  to  last 
for  only  15  minutes.  The  Graminales  includes  two  families: 
The  Graminaceae,  or  grass  family,  and  the  Cyperaceae  or  sedge 
family.  The  grass  family  is  by  far  the  more  important  and  is 
distinguished  by  the  usually  hollow  aerial  stems,  two  rows  of 
leaves  with  sheathing  bases  split  and  the  fruit  is  generally  a 
grain  (Fig.  284,  6).  The  sedges  are  largely  paludose  and  of 
little  value,  the  stems  and  leaves  being  often  silicified.  They 
usually  have  solid  stems,  three  rows  of  leaves  with  sheathing 
bases  entire,  more  simple  flowers  and  the  fruit  is  usually  an 
akene  (Fig.  285). 

132.  Arales,  the  Aroid  Order. — These  plants  are  largely  tropi- 
cal and  are  characterized  by  large  and  generally  net-veined  leaves 
and  showy  inflorescences  and  fruit  (Fig.  286).  They  present 
an  odd  series  of  striking  forms  quite  different  from  other  mono- 
cotyledons. Many  of  them  are  familiar  plants  owing  to  their 
extensive  cultivation  in  green-houses,  as  the  calla  lily  (Richardia), 
Caladium,  often  with  variegated  leaves,  and  the  gigantic  leaves  of 
Dracontium  and  Colocasia,  known  as  the  elephant  ear,  and  the 
curiously  perforated  leaves  of  the  Monstem.  Many  species  of 
Anthurium  are  climbers  and  reach  to  the  top  of  the  highest 
trees,  sending  out  with  great  regularity  from  the  successive  nodes, 
naked  branches  that  may  reach  the  ground  and  form  roots. 
The  order  is  represented  in  temperate  regions  by  a  few  very 
familiar  genera  as  jack-in-the-pulpit  (Arisaema),  skunk  cabbage 
(Spathyema) ,  golden  club  (Orontium),  sweet  flag  (Acorus),  water 
arum  (Calla).  The  duckweeds  that  float  upon  nearly  every 
pond  are  extremely  reduced  allies  of  the  order,  representing  the 
smallest  seed  plants  known,  Wolffia  being  an  oval,  rootless  plant 
scarcely  one  millimeter  in  diameter. 

The  inflorescence  is  usually  covered,  as  in  Typha,  with  a  bract 
known  as  the  spa  the.  This  organ  is  variously  colored  and  often 
the  most  attractive  feature  of  the  plant.  The  coloration  and  the 


DEVELOPMENT   OF    PLANTS 


411 


odors  and  glands  that  are  developed  in  some  of  the  forms  indicate 
that  these  plants  are  pollinated  by  insects.  The  flowers  are 
arranged  upon  a  fleshy  axis,  the  spadix,  and  in  the  simplest  forms 
are  quite  comparable  to  those  of  Typha,  consisting  of  one  or 
a  few  stamens  or  a  single  pistil  (Fig.  286,  B-D).  These  im- 


FIG.  286.  Inflorescence  of  the  Arales:  A,  habit  of  the  jack-in-the-pulpit. 
B,  diagram  of  the  inflorescence  with  spathe,  s,  opened  on  one  side  to  show 
the  spadix,  sp,  bearing  staminate  flowers  at  the  base.  C,  a  pistillate  flower 
consisting  of  a  single  naked  carpel.  D,  a  staminate  flower  of  four  two-lobed 
anthers.  E,  section  of  the  inflorescence  of  one  of  the  arums — p,  compart- 
ment containing  pistillate  flowers;  s,  staminate  flowers. 

perfect  flowers  may  be  developed  on  different  parts  of  the  same 
spadix  or  on  different  spadices.  In  more  advanced  types,  the 
flowers  are  perfect,  containing  one  or  several  pistils  that  may  be 
united  and  stamens  which  are  surrounded  by  a  very  rudimentary 


412  THE  ARALES 

perianth.  The  relation  of  these  plants  to  insects  is  a  very  inter- 
esting one.  The  brightly  colored  spathe  serves  as  an  allurement 
and  it  also  protects  the  microspores  against  wetting.  The  spores 
of  comparatively  few  plants  can  endure  water  and  it  will  be 
interesting  to  observe  in  the  following  lessons  the  devices  that 
appear  to  guard  against  this  danger.  The  spathe  also  affords 
shelter  to  the  insect  and  the  temperature  is  higher  within  the 
spathe  since  the  food  is  being  rapidly  oxidized  while  the  spores 
are  forming.  The  temperature  of  the  larger  spadices  may  ap- 
proach blood  heat,  being  10  to  25°  C.  higher  than  the  outside 
air.  Insects  are  not  slow  to  avail  themselves  of  these  advantages 
and  they  also  find  an  abundance  of  food  in  the  microspores  and 
glands  that  are  often  developed  upon  the  spathe.  Many  aroids 
are  characterized  by  the  foul  odors  of  decaying  flesh  and  it  is 
noteworthy  that  the  coloration  of  the  spathe  is  in  singular  har- 
mony with  these  odors,  resembling  the  hues  of  putrid  meat.  As 
a  result  of  these  variations,  the  lower  orders  of  insects  visit  these 
flowers  in  great  numbers  and  unwittingly  serve  in  the  transfer 
of  the  microspores;  250  carrion  beetles  and  over  1,000  midges 
have  been  reported  as  taken  from  a  single  spathe.  You  can 
readily  demonstrate  that  small  insects  are  attracted  to  the  spathe 
by  examining  the  inflorescence  of  the  skunk  cabbage  and  jack- 
in-the-pulpit. 

Common  Examples  of  the  Arales. — In  the  skunk  cabbage 
(Spathyema  foetida),  which  is  of  common  occurrence  along 
muddy  streams,  the  inflorescence  may  appear  as  early  as  Febru- 
ary, while  the  leaves  are  still  rolled  up  into  a  compact  bodkin  to 
enable  them  to  penetrate  the  soil.  Later  the  leaves  unroll  and 
become  very  large,  being  good  types  of  aroid  leaves  with  broad 
blades,  strongly  net- veined  and  evidently  adapted  to  abundant 
moisture  and  rapid  transpiration.  The  spathe  is  shell-like,  mot- 
tled with  purple,  green  and  yellow  and  encloses  a  fleshy  oblong 
spadix  that  is  entirely  covered  with  flowers.  You  will  find  that 
small  insects  are  attracted  to  these  spathes  in  great  numbers  and 
spiders  often  take  advantage  of  this  fact  and  spin  their  webs 
out  of  sight  in  the  darker  recesses  of  the  spathe.  In  this  plant, 
the  flowers  are  decidedly  in  advance  of  those  noted  in  Typha, 


DEVELOPMENT   OF   PLANTS  413 

being  perfect,  and  in  addition  containing  a  perianth  of  four 
sepals  which  arch  over  the  sporophylls.  The  crossing  of  the 
flowers  is  effected  in  a  variety  of  ways.  In  some  cases  the  in- 
conspicuous stigmas  first  push  out  from  between  the  sepals  so 
that  microspores  must  be  brought  from  another  earlier  flowering 
plant.  In  other  cases,  the  stamens  at  the  top  of  the  spadix  are 
first  extruded  and  shed  their  spores;  while  at  the  bottom  of  the 
spadix  the  reverse  condition  obtains,  the  stigmas  being  in  a  re- 
ceptive condition.  This  might  result  in  a  crossing  of  the  upper 
flowers  with  the  lower,  and  later,  when  the  lower  flowers  put 
out  their  stamens  and  the  upper  their  stigmas,  insects  could  effect 
a  reverse  crossing.  As  the  seeds  mature  the  spadix  and  sepals 
become  large  and  spongy,  enclosing  the  ovaries  which  are  finally 
set  free  by  the  decay  of  the  spadix,  as  fleshy  berries.  This  fruit 
is  admirably  adapted  to  the  aquatic  habit  of  these  plants  and 
readily  floats  in  the  water,  owing  to  the  spongy  outer  tissues,  and 
so  bring  about  the  distribution  of  the  seeds.  The  jack-in-the- 
pulpit  has  a  surer  method  for  effecting  a  crossing,  since  the 
sporophylls  are  usually  borne  on  separate  spadices.  You  will 
often  find,  however,  spadices  with  a  few  stamens  situated  just 
above  the  pistillate  flowers.  The  flowers  are  very  simple,  con- 
sisting of  a  single  naked  pistil  or  of  four  nearly  sessile  stamens 
(Fig.  286,  C-D).  The  fruit  is  a  shining  bright  red  berry. 

In  some  of  the  tropical  genera  the  insects  are  held  in  the 
spathe  by  hairs  that  extend  obliquely  downwards,  thus  permit- 
ting the  entrance  but  blocking  the  exit  of  the  insects  until  cross- 
ing of  the  flowers  has  been  effected.  In  one  of  the  species  of 
Arum  there  are  two  sets  of  hairs  that  divide  the  spathe  into  two 
compartments  containing  respectively  staminate  and  pistillate 
flowers  (Fig.  286,  E).  As  soon  as  the  spathe  opens  the  insect 
ladened  with  microspores  from  another  flower  makes  his  way  to 
the  lower  chamber,  where  the  stigmas  are  in  a  receptive  condi- 
tion. Here  he  is  held  a  prisoner  and  rubs  the  microspores  upon 
the  stigmas  as  he  wanders  about  in  the  compartment.  When  the 
microspores  in  the  upper  chamber  begin  to  discharge,  the  lower 
set  of  hairs  wither,  giving  him  entrance  to  this  chamber,  where 
he  feeds  upon  the  microspores  and  becomes  covered  with  thenu 


414 


THE   LILIALES 


Finally  the  upper  set  of  hairs  wither  and  he  is  free  to  leave  the 
spathe  and  repeat  his  work  in  another  inflorescence. 

133.  Liliales,  the  Lily  Order. — The  members  of  this  order 
comprise  nearly  5,000  species  that  are  widely  distributed  and 
extensively  cultivated  for  their  showy  flowers.  We  have  now 
reached  a  point  in  the  evolution  of  the  flower  where  it  has  become 
perfect  and  the  protective  spathe  of  preceding  orders  is  replaced 
by  a  well-developed  perianth.  The  floral  axis  also  becomes  short- 


FIG.  287.  The  fawn  lily,  Erythronium  americanum:  A,  habit  of  the  plant; 
B,  the  bulb,  showing  the  origin  of  the  stem  and  leaves  shown  in  A;  r,  run- 
ners that  penetrate  the  soil  forming  new  bulbs  at  their  tips.  C,  pistil  of 
three  carpels,  at  the  right  the  fruit,  a  capsule,  opening  to  scatter  the  seeds. 

ened  and  we  pass  from  the  spiral  series  of  flowers  with  a  vari- 
able number  of  organs  to  the  cyclic  flowers  with  a  definite 
number  of  organs  arranged  in  whorls.  In  the  lily  order  there 
are  five  whorls  of  organs  of  three  members  each,  the  stamens 
being  arranged  in  two  whorls.  Note  also  that  this  crowding 
usually  results  in  a  compound  pistil  of  three  carpels  (Fig.  287, 
C).  These  plants  are  largely  perennial,  with  underground 
stems  in  the  form  of  bulbs  or  rhizomes.  This  feature  adapts 


DEVELOPMENT   OF   PLANTS  415 

many  of  them  to  dry  and  steppe  regions,  from  which  source  many 
of  our  cultivated  lilies  have  been  obtained.  It  will  be  seen  that 
this  habit  of  storing  food  in  underground  stems  enables  these 
plants  to  develop  their  leaves,  flowers  and  fruit  during  the  short 
rainy  season,  after  which  the  entire  aerial  portion  withers  away 
and  their  life  lies  dormant  in  the  buried  stems.  This  habit  is 
equally  serviceable  if  these  plants  come  into  competition  with 
larger  forms,  as  in  forests  where  plants  with  bulbs  and  rhizomes 
may  complete  their  annual  growth  before  the  grosser  vegetation 
that  would  crowd  them  out  is  fairly  started  (see  page  44). 

(a)  The  Fawn  Lily,  Erythronium  americanum. — This  species 
may  be  examined  as  typical  of  the  order  (Fig.  287).  This  plant 
has  received  the  atrocious  name  of  adder's  tongue,  which  is 
offensive  and  far-fetched,  and  also  of  dog-tooth  violet,  although 
it  is  not  a  violet  at  all.  Burroughs  has  suggested  the  very  appro- 
priate name  of  trout  lily,  since  the  mottled  leaves  often  form 
conspicuous  beds  on  shady  banks  of  streams;  but  to  those  who 
have  experienced  the  spring  time  in  the  north  country,  the  term 
fawn  lily  seems  singularly  appropriate.  The  leaves  of  the  fawn 
lily  spring  from  deep-seated  bulbs  that  are  formed  in  a  peculiar 
way.  The  seed  germinates  on  the  surface  of  the  soil  and  forms 
a  very  small  bulb  and  a  single  grass-like  leaf.  During  each  suc- 
ceeding season  a  larger  leaf  and  bulb  are  formed,  and  when  of 
sufficient  size,  the  bulb  sends  out  one  or  more  runners  that  pene- 
trate the  soil  and  develop  new  bulbs  at  their  tips  (Fig.  287,  B). 
In  this  way  the  bulbs  become  deep-seated  and  rapidly  increase 
in  numbers,  and  after  several  years  they  attain  sufficient  size  to 
develop  two  leaves  and  a  flower.  The  mottled  leaves  have  the 
same  habit  of  rolling  up  in  emerging  from  the  ground,  as  noted 
in  the  skunk  cabbage.  It  is  to  be  observed  that  the  position 
assumed  by  the  mature  leaf  of  many  plants  is  often  strikingly 
correlated  with  the  extent  of  the  root  system.  In  the  Liliales 
generally,  which  do  not  have  extensive  lateral  roots,  the  hang  of 
the  leaves  is  such  as  to  direct  the  water  that  falls  upon  them 
towards  the  center  of  the  plant,  a  feature  doubtless  of  consider- 
able advantage  to  plants  living  in  semi-arid  regions.  The  flower 
is  of  a  decidedly  higher  type  than  any  previously  studied  and  it 


4i6  THE   LILIALES 

presents  several  features  of  special  interest.  The  flower  is  later- 
ally placed  or  slightly  pendulous.  The  perianth  is  conspicuously 
developed,  consisting  of  two  whorls  of  three  numbers  each  though 
these  organs  are  not  as  yet  fully  differentiated  into  calyx  and 
corolla.  The  stamens  are  also  arranged  in  two  whorls  of  three 
members  each  and  the  three  carpels  form  a  single  whorl  appearing 
as  a  compound  pistil.  The  members  of  these  whorls  alternate 


FIG.  288.  Lower  forms  of  the  Liliales:  A,  a  common  rush,  Juncus,  show- 
ing grass-like  appearance  of  the  stem  and  inflorescence.  B,  a  flower  enlarged, 
showing  a  lily  type.  C,  white  hellebore,  Veratrum.  D,  flower  enlarged, 
showing  the  partial  coherence  of  the  carpels  of  this  primitive  type  of  the 
Liliales. 


DEVELOPMENT  OF  PLANTS  417 

with  each  other,  so  that  the  flower  has  three  planes  of  symmetry. 
At  maturity  the  walls  of  the  ovary  become  papery  and  split 
down  the  side,  thus  freeing  the  seeds.  This  form  of  fruit  is 
known  as  the  capsule,  and  is  of  very  common  occurrence  in  the 
order  (Fig.  287,  C).  The  development  of  a  conspicuous  perianth 
is  a  noteworthy  departure.  This  structure  protects  the  micro- 
spores  and  replaces  the  bracts  and  spathe  of  preceding  orders, 
but  owing  to  its  peculiar  coloration  and  form,  it  also  serves  as 
an  attraction  to  special  kinds  of  insects.  In  the  preceding 
forms  that  attracted  insects,  the  inducements  were  usually 
in  the  nature  of  shelter  and  foods,  which  were  largely  the  micro- 
spores  that  were  offered  freely  to  all.  Such  flowers  are  princi- 
pally visited  by  a  low  order  of  stupid  flies  and  beetles  that  are 
rather  promiscuous  feeders  and  are  quite  as  likely  to  go  from 
a  flower  of  one  species  to  that  of  a  different  kind,  or  to  some 
.  other  object  and  so  defeat  the  principal  object  of  the  flower. 
With  the  development  of  the  perianth,  however,  the  flower  is 
equipped  with  a  device  that  primarily  serves  to  exclude  these  less 
desirable  visitors  and  to  attract  the  more  intelligent  ones.  In 
Erythronium  the  perianth  is  in  a  horizontal  position,  so  that  it 
offers  a  natural  landing  place  for  the  insect,  and  the  inducement 
is  a  sugary  solution  secreted  by  nectar  glands  concealed  at  the 
base  of  the  perianth.  It  is  an  easy  task  for  the  more  intelligent 
long-tongued  insects,  like  the  bees,  wasps  and  butterflies,  to  reach 
the  nectar  in  this  type  of  flower.  Such  insects  come  to  know  by 
experience  how  to  gather  the  food  from  a  particular  form  of 
flower  and  consequently  they  will  often  confine  their  attention  to 
a  single  species  during  their  entire  flight;  consequently  the  flower 
by  excluding  the  less  intelligent  and  slothful  insects  is  more  cer- 
tain of  being  properly  crossed.  The  development  of  nectar  and 
also  of  odor  glands  is  among  the  important  variations  that 
appear  in  the  evolution  of  the  flower.  It  is  chiefly  by  means  of 
the  perfumes  derived  from  these  organs  that  the  insect  is  directed 
to  the  flowers.  The  coloration  is  also  of  service,  when  the  insect 
is  near  to  the  flower,  thus  supplementing  the  perfume  glands  by 
directing  him  to  the  proper  entrance.  Some  of  the  larger  and 
more  brightly  colored  members  of  this  and  other  orders  appa- 


4i8  THE   LILIALES 

rently  depend  entirely  upon  the  attraction  of  their  colors,  while 
other  forms  rely  upon  perfumes,  the  flowers  being  small  and  in- 
conspicuous and  often  hidden.  So  we  have  reached  a  point  in 
the  evolution  of  the  flower  where  it  presents  a  number  of  varia- 
tions that  are  adapted  and  of  benefit  to  special  kinds  of  insects. 
Insects  have  likewise  varied  and  some  have  become  of  special  use 
to  flowers  owing  to  their  peculiar  form.  In  this  way  certain  in- 
sects and  flowers  have  become  dependent  upon  each  other,  and 
as  a  result  of  their  mutually  beneficial  variations,  the  flowering 
plants  and  insects  have  greatly  increased  in  numbers  and  exceeded 
all  other  groups  in  the  plant  and  animal  kingdoms  (p.  388). 

(b)  Other  Forms  of  the  Lily  Order. — This  order  comprises 
an  extensive  series  of  plants  that  present  nearly  all  the  variations 
that  occur  in  the  flower.  At  the  bottom  of  the  series  is  the  Rush 
family  with  grass-like  stems  and  leaves  (Fig.  288,  A,  B).  The 
flower  has  the  same  structure  as  noted  in  Erythronium,  but  the 
perianth  is  composed  of  small  scale-like  leaves  destitute  of  nectar 
glands,  and  as  you  would  expect,  is  wind  pollinated.  The  Bunch- 
flower  family  is  a  somewhat  higher  type.  The  perianth  is  some- 
what larger  and  more  delicate  then  in  the  rushes  and  the  carpels 
are  often  but  partially  fused  (Fig.  288,  C,  D).  The  flowers  gain 
in  conspicuousness  by  being  associated  in  compact,  inflores- 
cences, but  do  not  develop  as  a  rule  the  bright  colors  character- 
istic of  large  flowers.  The  family  is  represented  by  Tofieldia, 
swamp  pink  (Helonias),  blazing  star  (Chamaelirium) ,  Zygade- 
nus,  hellebore  (Veratrum),  bellwort  (Uvularia) ,  autumn  crocus, 
etc.  The  Lily  of  the  Valley  family  is  characterized  by  rather 
larger  flowers  than  those  of  the  previous  family,  and  they  are  less 
clustered  and  often  provided  with  tubular  perianths  owing  to 
the  mass  growth  of  the  perianth  segments.  The  fruit  is  a  berry, 
unlike  the  majority  of  the  families  of  this  order.  Here  belong 
the  asparagus,  the  false  and  true  Solomon's  seal,  Indian  cucum- 
ber, Trillium,  and  the  closely  related  Smilax,  etc.  The  Lily 
family  includes  nearly  one  half  of  the  members  of  the  order  and 
occupies  a  medium  position  in  the  evolution  of  the  group.  The 
flowers  are  large  and  of  the  Erythronium  type.  Here  belong 
some  of  the  most  showy  of  our  native  and  cultivated  plants;  the 


DEVELOPMENT   OF   PLANTS 


419 


day  lily,  Erythronium,  tulips,  hyacinths,  lilies,  Fritillaria,  onion, 
aloes  and  Spanish  bayonets  (Yucca)  of  arid  regions.  Among 
the  higher  genera  of  the  Lily  family  the  organs  of  the  perianth 
often  form  a  tubular  structure.  In  the  Amaryllis  family  we 
find  the  same  type  of  flower  and  fruit  as  in  the  Lily  family,  but 
the  basal  growth  of  the  receptacle  has  enveloped  the  ovary  so 
that  the  flower  has  become  epigynous  (Fig.  289,  A).  This 


FIG.  289.  Advanced  forms  of  the  Liliales:  A,  Narcissus  with  inferior 
ovary,  o.  The  six  sepals  cohere  at  their  base,  forming  a  tube  and  they  also 
develop  an  outgrowth  at  the  mouth  of  this  tube,  which  surrounds  the  anthers 
and  stigma  like  a  cup.  B,  flower  of  Iris.  C,  section  of  same,  showing  in- 
ferior ovary,  o;  stigma,  g;  and  anther,  a. 

large  family  furnishes  a  number  of  showy  flowers,  as  the  jonquil 
and  daffodil  ( Narcissus) ,  amaryllis,  snowflake,  Crinum,  star 
grass,  century  plant,  etc.  The  flowers  of  some  genera  become 
irregular  through  the  unequal  development  of  certain  leaves  of 
the  perianth.  The  Iris  family  marks  the  culmination  of  the 
variations  noted  in  the  order.  The  types  of  flower  and  fruit  are 
like  those  of  the  previous  family,  but  the  crowding  on  the  recep- 
tacle has  resulted  in  the  obliteration  of  one  whorl  of  stamens. 
The  structure  of  the  flowers  often  shows  a  series  of  variations  that 
adapt  them  to  insect  visitors  and  crossing,  as  is  well  illustrated 


420  THE  SCITAMINALES 

in  the  iris  (Fig.  289,  B,  Q.  The  styles  have  a  very  unusual 
form,  resembling  a  leafy  organ  with  the  stigma  on  the  upper  side 
of  a  small  projecting  shelf.  Beneath  each  of  these  curving  styles 
is  a  stamen.  The  insect  naturally  alights  upon  the  broad  leaf 
of  the  outer  whorl  of  the  perianth  and  the  peculiar  coloration  of 
this  leaf  probably  directs  him  to  the  nectar  secreted  at  the  base 
of  the  perianth.  In  reaching  this  food  he  crowds  down  into  the 
tube  and  rubs  off  the  microspores  upon  his  back.  In  leaving  the 
flower  he  cannot  hit  the  stigma,  but  in  visiting  another  flower  it 
can  readily  be  seen  that  he  will  deposit  some  of  the  spores  upon 
the  stigma  situated  upon  the  top  of  the  shelf,  thus  effecting  a 
crossing.  This  order  includes  a  large  number  of  showy  culti- 
vated forms,  as  the  iris,  fleur-de-lis,  crocus,  gladiolus,  Friesia, 
blue-eyed  grass,  etc. 

134.  The  Scitaminales. — This  order  is  confined  largely  to  the 
tropics  and  includes  such  familiar  plants  as  the  banana,  the  trav- 
eler's tree,  ginger  plant,  canna,  etc.  It  is  mentioned  here  as  fur- 
nishing an  interesting  illustration  of  the  variations  that  often 
appear  in  epigynous  flowers  as  a  result  of  the  crowding  of  the 
organs  upon  the  receptacle.  Tendencies  toward  irregularity  ap- 
peared in  the  epigynous  families  of  the  Liliales,  but  in  this 
order  these  variations  become  very  marked  and  make  less  abrupt 
the  transition  to  the  next  order.  The  flowers  of  the  banana  are 
suggestive  of  the  Amaryllis  family,  though  somewhat  irregular* 
All  of  the  perianth  leaves  but  one  are  united  and  only  five  per- 
fect stamens  are  developed.  These  flowers  are  united  into  groups 
in  the  axils  of  bracts  that  form  large  buds  at  the  ends  of  the  stem. 
As  the  bud  elongates,  the  basal  or  lower  bract  is  first  curved 
back  from  the  bud,  thus  exposing  the  flower  cluster,  and  this 
expansion  goes  on  until  all  the  flowers  are  exposed.  The  lowest 
flowers  in  the  inflorescence  are  imperfect,  containing  only  pistils, 
the  central  are  perfect  and  the  upper  ones  bear  only  stamens. 
Only  the  ovaries  of  the  lower  flowers  of  the  inflorescence  develop, 
each  "hand"  of  bananas  representing  the  matured  ovaries  of  the 
flowers  in  the  axil  of  the  bract.  Singularly,  seeds  do  not  develop 
in  the  edible  banana,  although  you  can  see  minute  dark-colored 
ovules  forming  three  radiating  lines  in  a  cross-section  of  the 


DEVELOPMENT  OF   PLANTS 


421 


fruit.  The  ability  of  these  plants  to  propagate  themselves  by 
means  of  buds  developed  on  the  underground  rhizomes  may  have 
resulted  in  the  loss  of  the  seed  habit.  A  great  many  plants  are 
so  successful  in  propagating  themselves  by  buds,  bulbs,  runners, 
etc.,  that  they  have  ceased  to  produce  seed.  In  the  higher  mem- 
bers of  the  order,  as  in  the  ginger  and  canna  families,  the  flowers 
become  very  irregular  through  the  unequal  development  of  the 
leaves  of  the  perianth  or  in  some  forms  on  account  of  the  abor- 


FIG.  290.  Flower  of  Canna,  the  showy  petal-like  organs  being  modified 
stamens  (staminodia)  while  the  perianth  proper,  p,  is  reduced  to  green  bract- 
like  organs:  /,  labellum;  an,  anther  on  petal-like  organ;  s,  stigma;  o,  ovary. 
B,  section  of  flower,  showing  ovary,  o,  the  modified  stamen,  an,  and  stigma, 
s,  the  other  organs  being  removed. 

tion  or  transformation  of  all  the  stamens  save  one  into  petal-like 
organs  termed  staminodia.  Usually  one  of  the  leaves  of  the 
perianth,  or  one  or  more  of  the  staminodia,  are  highly  modified 
and  known  as  the  labellum.  This  organ  is  so  placed  as  to  afford 
a  natural  landing  place  for  the  insects  visiting  the  flower  and  also 
to  necessitate  crossing  (Fig.  290).  Observe  a  bee  visiting  the 
flowers  of  the  canna  and  determine  the  significance  of  the  position 
and  movement  of  the  labellum  and  its  relation  to  the  stamen  and 
stigma. 

135.  Orchidales,   the    Orchid   Order. — The   orchids   are   the 


422  THE   ORCHIDALES 

highest  group  of  the  monocotyledons  and  their  flowers  are  a 
source  of  wonder  and  admiration,  owing  to  the  singular  beauty 
and  delicacy  of  their  mechanical  construction.  Variation  in  this 
order  has  occurred  on  a  gigantic  scale,  resulting  in  a  larger  num- 
ber of  species  (over  7,000)  than  is  found  in  any  of  the  preceding 
orders.  Nevertheless  these  elaborate  variations  have  not  been 
very  successful  in  enabling  them  to  compete  with  other  plants, 
and  as  a  result  the  orchids  are  rather  rare  and  not  at  all  com- 
parable in  number  of  individuals  with  the  lilies  and  grasses. 
Though  more  widely  distributed  than  any  of  the  other  monocoty- 
ledons, their  variations  have  adapted  them  as  a  rule  to  peculiar 
conditions.  They  are  especially  abundant  in  the  mountainous 
districts  of  the  tropics,  where  they  more  commonly  appear  as 
epiphytes  upon  the  trunks  of  trees  and  in  the  crevices  of  rocks. 
Such  conditions  are  met  by  the  development  of  a  thick  mantle 
of  cells  about  the  aerial  roots,  the  velamen,  which  absorbs  the 
moisture  from  the  air  and  doubtless  the  enlargement  of  the  leaf 
base  in  some  of  these  plants  into  a  bulbous  storage  organ  enables 
them  to  anticipate  in  this  way  the  heavy  demands  that  will  be 
made  upon  them  in  the  flowering  season  (Fig.  291).  The  ter- 
restrial forms  are  largely  parasitic  or  saprophytic  and  associated 
with  mycorrhiza,  and  this  has  resulted  in  some  of  the  forms  in  the 
suppression  of  various  organs  of  the  plant  as  the  primary  root  or 
even  of  the  entire  root  system,  as  is  illustrated  in  the  coral  root 
orchid,  where  the  leaves  have  also  become  reduced  to  mere  scales 
and  the  chlorophyll  has  disappeared.  The  dependence  upon 
fungal  life  has  reached  such  a  stage  in  some  forms  that  the  seeds 
do  not  germinate  unless  hyphae  come  in  contact  with  them 
(p.  69).  The  organs  of  the  flowers  are  subject  to  such  remark- 
able variations  that  the  various  parts  may  appear  at  first  some- 
what difficult  to  recognize.  It  is  evident  that  the  same  line 
of  variation  noted  in  the  higher  families  of  the  Liliales  has  been 
continued,  for  the  perianth  is  arranged  upon  a  compound  ovary, 
but  its  parts  are  often  sharply  differentiated  into  calyx  and 
corolla.  The  sepals  and  petals  comprising  these  two  whorls 
differ  greatly  in  form,  but  especially  to  be  noted  is  the  oddly  con- 
structed petal  known  as  the  labellum  (Figs.  292,  293,  Q.  This 


DEVELOPMENT  OF  PLANTS  423 

organ  is  the  most  striking  feature  of  the  orchid  and  it  assumes 
an  almost  endless  variety  of  forms  and  colorations,  being  a  good 
illustration  of  Wallace's  law  that  the  most  highly  modified  part 
shows  the  greatest  variation  in  coloration.  In  Cypripedium  (Fig. 
292,  A)  the  labellum  assumes  the  form  of  a  moccasin,  in  other 
genera  it  resembles  a  vase,  boat,  tongue,  body  of  insect,  etc.  It 


FIG.  291.  An  epiphytic  orchid  growing  upon  the  branch  of  a  tree.  The 
coarse  roots,  r,  are  surrounded  by  a  mantle  of  cells  which  takes  up  the  mois- 
ture from  the  atmosphere,  b,  storage  organs  formed  from  the  base  of  the 
leaves,  enabling  the  plant  to  produce  flowers  and  fruit.  The  smaller  stalks, 
a,  are  the  shriveled  remains  of  these  organs  after  flowering  and  fruiting. 

is  entire  or  variously  lobed,  slit,  fringed  and  often  prolonged  into 
a  tube  for  the  concealment  of  nectar.  The  stamens  and  stigmas 
are  reduced  in  number  and  greatly  modified,  the  former  organs 
usually  being  reduced  to  one  and  so  fused  with  the  style  that  the 
anther  is  sessile  upon  it  (Fig.  292,  B).  One  or  two  of  the  stig- 
mas are  generally  modified  into  a  mucilage-secreting  organ,  called 
the  rostellum  (Fig.  293,  A).  The  microspores  are  usually  united 


424 


THE   ORCHIDALES 


into  a  waxy  mass,  pollinium  (pi.  pollinia)  which  is  attached  to 
a  sticky  part  of  the  rostellum  (Fig.  293,  B,  C}.  The  remarkable 
feature  about  these  gaudy  flowers  is  the  relation  that  the  label- 
lum,  anther,  rostellum  and  stigma  sustain  to  each  other.  The 
position  of  these  organs  is  such  that  an  insect  visiting  the  flower 
touches  with  some  part  of  his  body  the  sticky  part  of  the  rostel- 
lum and  the  pollinia  are  thus  made  fast  to  him  and  carried  to  the 
stigma  of  another  flower.  These  devices  are  so  elaborate  in 
many  orchids  that  the  microspores  can  only  reach  the  stigma 
through  the  agency  of  an  insect.  In  the  lower  types  of  orchids, 
as  the  moccasin  flower  (Fig.  292,  B),  a  somewhat  different 


FIG.  292.  A  simple  type  of  the  Orchidales:  A,  the  moccasin  flower,  Cy- 
pripedium — /,  labellum;  p,  the  two  unmodified  petals;  s,  sepals,  two  being 
united  below  the  labellum;  b,  bract,  partially  concealing  the  inferior  ovary. 
B,  section  of  the  flower — /,  labellum;  s,  stigma;  an,  anther;  st,  shield-like 
sterile  stamen  covering  the  two  anthers  and  stigma;  o,  ovary;  b,  bract. 

arrangement  is  found.  The  bee  enters  the  opening  in  the  upper 
part  of  the  labellum  and  feeds  upon  the  glands  distributed  along 
the  bottom.  In  leaving  the  flower  he  forces  his  way  through 
the  small  opening  on  either  side  of  the  style  and  so  he  first  comes 
into  contact  with  the  stigma  and  later  with  the  anthers,  which  are 
two  in  number  and  located  back  of  the  stigma.  In  this  flower 
the  anthers  are  surrounded  by  a  sticky  mass  and  the  microspores 
are  thus  fastened  to  the  insect's  body  to  be  carried  to  another 
flower.  In  the  higher  types  of  orchids,  the  insect  probing  for  the 
nectar  touches  the  sticky  discs  of  the  rostellum,  to  which  the 


DEVELOPMENT  OF  PLANTS 


425 


pollinia  are  attached,  and  in  this  way  they  are  fastened  to  his 
body  and  drawn  out  of  the  open  anthers  when  he  leaves  the 
flower  (Fig.  293,  C).  In  some  cases  the  pollinia  quickly  curve 
after  being  withdrawn  from  the  anther,  with  the  result  that  this 
change  of  position  brings  them  into  line  with  the  stigma  of  the 
next  flower  visited.  In  other  genera  certain  cells  are  irritable 
and  in  a  high  state  of  tension.  A  touch  causes  an  explosion  that 
results  in  hurling  out  the  pollinia  which  always  land  on  the  end  to 
which  the  sticky  disc  is  attached  and  so  become  fastened  to  the 


FIG.  293.  Higher  type  of  the  Orchidales:  A,  flower  of  Orchis— I,  label- 
lum;  p,  the  two  unmodified  petals;  s,  sepals;  r,  rostellum  to  which  the  two 
pollen  masses,  pollinia,  in  the  two-lobed  anther,  an,  are  attached  by  sticky 
discs;  st,  stigma.  B,  enlarged  lateral  view  of  the  anther  which  has  opened, 
exposing  the  pollinia.  C,  one  of  the  pollinia  withdrawn  from  the  anther  show- 
ing adhesive  disc  at  base  which  is  attached  to  the  rostellum.  At  right  a 
pollinium  enlarged,  showing  the  small  masses  of  microspores,  massula,  that 
may  be  separately  detached  from  the  pollinium. — After  Warming. 

insect.     (See  Darwin's  Fertilization  of  Orchids  for  a  discussion 
of  the  multiplicity  of  these  arrangements.) 

The  seeds  are  among  the  most  rudimentary  of  the  Spermato- 
phyta,  consisting  of  an  undifferentiated,  few-celled  embryo  with- 
out endosperm  and  surrounded  by  a  delicate  bladder-like  integu- 
ment. They  are  exceedingly  small  and  produced  in  enormous 
numbers;  in  fact,  the  seeds  of  the  rattlesnake  orchid,  Peramium, 
weigh  but  two  millionths  of  a  grain  each  and  float  in  the  air  like 
28 


426  DICOTYLEDONES 

dust  particles.  The  fringed  orchis  ( Blephariglottis) ,  Arethusa, 
ladies'  tresses  (Gyrostachys) ,  rattlesnake  plantain  (Peramium), 
grass  pink  (Lima dorum) ,  rose  pogonia,  showy  orchis  (Galeorchis) , 
moccasin  flower  (Cypripedium)  are  among  the  common  and 
more  showy  of  our  native  orchids. 

Class  B.    Dicotyledones 

136.  General  Characters. — The  structure  of  these  plants  is 
more  complex  than  that  of  the  monocotyledons  and  their  varia- 
tions have  been  more  extensive  and  successful,  over  100,000 
species  being  known.  For  this  reason  they  are  adapted  to  a 
greater  range  of  conditions  and  have  become  the  dominant  plants, 
forming  the  conspicuous  and  characteristic  features  of  the  vege- 
tation of  the  earth. 

The  dicotyledons  may  be  short-lived  annual  plants,  or  per- 
ennials, and  they  include  a  great  variety  of  climbers,  epiphytes, 
parasites,  and  saprophytes  and  comparatively  few  aquatics.  The 
most  extreme  forms  of  xerophytes  are  also  found  in  this  group. 
The  leaves  are  highly  differentiated,  usually  consisting  of  a 
blade  and  petiole  which  is  often  associated  with  small  leaf-like 
organs,  the  stipules  (Fig.  294).  The  blade  varies  in  form  and 
is  often  characterized  by  teeth  and  various  forms  of  lobing. 
This  is  due  to  the  fact  that  the  veins  usually  differ  from  those  of 
the  monocotyledons  in  that  they  repeatedly  branch,  becoming 
smaller  and  smaller  and  thus  forming  a  network  or  reticulated 
venation  with  free  ends  on  the  margins  and  other  parts  of  the  leaf. 
The  stems  are  markedly  different  from  the  previous  class,  owing 
to  the  arrangement  of  the  vascular  tissues  in  a  circle  about  the 
pith  (p.  76)  and  the  formation  of  a  cambium  cylinder  which 
brings  about  an  increase  in  the  diameter  of  the  stem  (Fig.  295). 
This  arrangement  gives  the  plant  a  great  advantage,  permitting 
the  extensive  system  of  branching  that  characterizes  the  group 
and  the  consequent  increase  in  the  display  of  foliage. 

The  flower  is  subject  to  the  same  modifications  as  noted  in  the 
monocotyledons.  Among  the  lower  orders,  the  flowers  are  quite 
as  simple  as  those  of  the  primitive  monocotyledons  and  the  de- 
velopment of  imperfect  and  wind  pollinated  flowers  is  of  common 


DEVELOPMENT   OF  PLANTS 


427 


occurrence.  The  spiral  arrangement  of  the  numerous  organs 
of  the  flower  will  also  be  noted.  The  majority  of  the  orders  of 
the  dicotyledons,  however,  are  characterized  by  perfect,  cyclic 
flowers  and  the  various  sets  of  organs  usually  consist  of  four  or 
five  members  each.  The  perianth,  when  present,  is  generally 
differentiated  into  a  green  calyx  and  a  variously  colored  corolla. 


FIG.  294.  FIG.  295. 

FIG.  294.  Leaf  of  white  birch,  the  blade,  b,  traversed  by  a  network  of 
veins  that  end  in  free  branches  (the  margin  irregularly  toothed  or  dentate) 
and  supported  upon  a  petiole,  p. 

FIG.  295.  Diagram  of  a  cross-section  of  a  stem  of  black  oak  four  years 
old:  p,  pith;  I,  2,  3,  4,  annual  rings  of  xylem;  c,  cambium  cylinder;  ph,  phloem; 
ct  cortex;  ck,  cork;  m,  medullary  rays. 


The  stamens  are  more  frequently  arranged  in  one  or  two  whorls, 
equalling  or  twice  the  number  of  the  sepals,  and  the  pistils  usually 
form  a  single  whorl,  equalling  or  less  than  the  number  of  sepals. 
The  crowding  of  the  various  organs,  and  the  lateral  growth  of 
the  receptacle  results  in  the  reduction  in  the  number  of  organs 
and  in  their  mass  growth  so  that  in  the  higher  types  the  calyx 
and  corolla  become  more  or  less  tubular  and  the  carpels  form  a 
compound  ovary.  The  basal  growth  of  the  receptacle  also  causes 
the  frequent  occurrence  of  the  perigynous  and  epigynous  types  of 
flowers.  Irregularity  of  the  flower  is  more  common  than  in  the 


428 


EMBRYO   OF   DICOTYLEDONES 


monocotyledons  and  this  feature  will  be  seen  to  characterize  the 
higher  members  of  many  of  the  orders. 

The  most  characteristic  feature  of  the  dicotyledons  is  the 
embryo  which  usually  consists  of  a  root,  stem,  and  two  laterally 
attached  cotyledons.  The  region  of  the  stem  above  the  attach- 
ment of  the  cotyledons  is  known  as  the  epicotyl  and  frequently  ap- 
pears as  a  minute  bud,  the  plumule.  The  region  of  the  stem  below 
the  cotyledons,  the  hypocotyl,  terminates  in  the  root  (Fig.  296). 


FIG.  296.  Structure  of  dicotyledonous  seeds:  A,  nearly  mature  seed  of 
Lepidium.  The  embryo  consists  of  the  hypocotyl,  hy,  ending  below  in  the 
root,  r,  and  the  root  cap  and  above  the  epicotyl,  pi.  Two  cotyledons,  c\ 
arise  laterally  from  the  stem;  /,  funiculus;  mi,  micropyle;  in,  integuments, 
en,  remains  of  endosperm.  B,  section  of  seed  of  water  lily — e,  embryo  with 
two  cotyledons  attached  laterally  to  the  minute  stem  of  the  embryo  and  sur- 
rounded by  a  layer  of  endosperm  cells;  mg,  sporangial  cells  or  perisperm;  i, 
integument. 

The  elongation  of  the  basal  portion  of  the  hypocotyl  frees  the 
root  from  the  seed  and  the  growth  of  the  upper  region  of  the 
hypocotyl  pushes  up  into  the  air  the  cotyledons  and  growing  point 
or  epicotyl.  The  formation  of  the  stem  is  sometimes  due  to 
the  elongation  of  the  epicotyl  alone,  the  cotyledons  frequently 
containing  the  storage  foods  for  the  nourishment  of  the  young 
plant  and  remaining  buried  in  the  seed. 


DEVELOPMENT  OF   PLANTS  429 

In  endospermous  seeds  it  should  be  noted  that  the  cotyledons 
are  not  developed  as  digestive  and  absorbing  organs  as  observed 
in  the  monocotyledons  (page  401).  The  primary  root  often  per- 
sists, forming  the  main  or  tap  root  of  the  plant,  which  manner  of 
growth  is  not  so  common  in  the  monocotyledons.  The  Dicoty- 
ledons include  two  rather  distinct  series:  a.  The  Choripetalae, 
distinguished  by  their  free  petals  or  lack  of  perianth,  b.  The 
Sympetalae,  with  united  petals. 

Series  a.     Choripetalae 

137.  General  Characters. — This  group  comprises  about  61,000 
species  and  includes  the  majority  of  the  trees  and  shrubs  found 
in  the  temperate  regions.     The  flowers  as  a  rule  are  of  a  simple 
type  but  exceedingly  variable  in  structure,  so  that  it  is  not  pos- 
sible to  separate  them  into  so  sharply  characterized  orders  as  in 
case  of  the  monocotyledons,  or  in  the  following  group  of  Sym- 
petalae.    Furthermore,  these  orders  doubtless  represent  many 
parallel  lines  of  development  that  are  imperfectly  understood. 
For  this  reason,  the  order  of  the  presentation  in  the  following 
pages  does  not  attempt  to  represent  the  real  relationship  of  the 
groups. 

138.  Salicales,  the  Willow  and  Poplar  Order.— The  willows 
are  almost  universally  distributed  along  water  ways,  as  though 
demanding  for  their  existence  only  light  and  water.     The  poplars 
are  less  restricted  and  some  can  endure  moderately  arid  condi- 
tions.    Few  plants  have  greater  vitality.     A  bit  of  a  twig  and 
often  a  portion  of  a  root  is  capable  of  developing  buds  and  so 
starting  the  shoot.     Trees  are  often  pointed  out  that  have  origi- 
nated through  the  careless  sticking  of  a  twig  in  the  soil  and 
several  of  the  willows  are  naturally  propagated  by  their  twigs 
which  are  easily  broken  off  by  the  winds.     Their  unusual  ability 
to  form  numerous  buds  is  well  shown  in  the  pollarded  willows 
where  the  branches  have  been  cut  back  and  a  large  number  of 
shoots  develop  about  the  wound.     Perhaps  the  stimulus  of  the 
wound  also  awakens  some  of  the  dormant  buds  (page  75).     The 
flowers,  as  in  Typha,  are  arranged  on  an  elongated  axis  forming 
a  compact  inflorescence  known  as  an  ament  or  catkin  (Fig.  297, 


430 


THE  SALICALES 


C,  E).     The  distinguishing  feature  of  this  kind  of  flower  cluster 
is  seen  in  the  scale  or  bract  that  protects  each  flower  (Fig.  297, 

D,  F).     The  aments  and  also  the  leaves  are  concealed  in  buds 
that  are  peculiar  in  that  they  are  protected  by  a  boat-shaped 
scale  (Fig.  297,  A).     These  plants  flower  very  early  in  the  spring 
and  as  the  ament  emerges  from  the  bud  the  overlapping  bracts 
with  their  hairy  coats  form  the  "pussy  willow"  stage  of  the 


B         E         F 

FIG.  297.  Flowers  and  seed  of  the  willow  (Salix):  A,  winter  appearance 
of  a  flowering  twig,  each  boat-shaped  scale  concealing  an  ament.  B,  pussy 
willow  stage  of  flowering,  the  aments  emerging  from  the  scales  and  exposing 
the  hairy  bracts  that  conceal  the  flowers.  C,  ament  of  pistillate  flowers  in 
full  bloom.  D,  pistillate  flower,  consisting  of  a  compound  pistil  of  two  car- 
pels— b,  bract;  n,  nectar  gland.  E,  ament  of  staminate  flowers.  F,  stam- 
inate  flower  of  two  stamens.  G,  ament  of  mature  pistils  which  are  opening 
to  discharge  the  seeds.  H—I,  successive  stages  in  the  opening  of  the  pistil. 
J,  a  seed  with  circle  of  hairs  at  base  forming  a  parachute  for  dissemination. 

inflorescence  (Fig.  297,  B).  Soon  the  bracts  spread  apart,  ex- 
posing the  flowers  which  are  nearly  as  primitive  as  the  simplest 
of  the  monocotyledons,  being  without  perianth,  imperfect  and 
usually  the  two 'kinds  of  sporophylls  are  arranged  on  different 
plants.  The  pistil  is  compound  and  composed  of  two  carpels, 
each  of  which  contains  numerous  ovules,  so  that  the  pistillate 


DEVELOPMENT  OF   PLANTS  431 

flowers  are  not  as  primitive  as  those  of  the  cat-tails,  where  the 
pistil  is  simple  (Fig.  297,  D).  The  staminate  flowers  show  the 
same  primitive  characters  (Fig.  297,  F),  consisting  of  one  or 
more  naked  stamens.  The  absence  of  showy  perianth,  the  ex- 
travagant production  of  microspores  and  the  formation  of  the 
flowers  before  the  leaves  become  large  and  so  interfere  with  the 
distribution  of  the  microspores  are  all  characteristics  of  wind  polli- 
nated flowers.  It  is  noteworthy,  however,  that  nectar  glands 
are  developed  in  the  flowers  of  the  willow  (Fig.  297,  D,  n)  and 
that  the  microspores  are  sticky.  Perhaps  we  have  here  an  illus- 
tration of  one  of  the  earliest  variations  of  the  flower  that  served  as 
an  allurement  to  insects.  Certainly,  the  nectar  glands  and  the 
conspicuous  display  of  microsporophylls  are  a  very  efficient 
attraction,  as  is  attested  by  the  variety  of  insects  that  swarm 
about  the  aments. 

The  most  efficient  factor  in  the  distribution  of  the  willows  and 
poplars  is  found  in  the  seed.  The  pistils  mature  in  the  early 
summer,  when  the  two  carpels  spread  apart  (Fig.  297,  G-I), 
permitting  the  discharge  of  the  seeds,  which  are  provided  with 
a  circle  of  hair  at  the  base  (Fig.  297,  J).  This  parachute  is 
not  so  nicely  constructed  as  in  Typha,  but  it  is  so  efficient  that 
myriad  numbers  of  minute  seeds  are  carried  a  considerable 
distance  and  cover  everything  in  the  neighborhood  of  the  trees 
with  their  lint-like  masses.  The  name  cottonwood  is  popularly 
applied  to  several  of  the  poplars  because  of  the  cotton-like  clusters 
of  seeds  that  emerge  from  their  aments. 

139.  Fagales,  the  Beech  Order. — This  order  includes  many 
of  the  most  important  hard-wood  trees  of  the  temperate  regions, 
comprising  the  family  of  the  birches,  with  such  representatives 
as  the  American  hornbeam  (Carpinus),  hop  hornbeam  (Ostrya), 
hazel  (Corylus),  birch  (Betula),  alder  (Alnus)  and  the  Beech 
family,  which  includes  the  chestnut  (Castanea) ,  beech  (Fagus) 
and  oak  (Quercus).  The  inflorescence  is  more  commonly  an 
ament  as  in  the  preceding  order  (Figs.  298,  A ;  299,  A),  although 
in  the  hazel  and  in  the  Beech  family  only  one  or  a  few  pistils  are 
developed  in,  the  bud-like  clusters  of  overlapping  bracts  (Fig. 
299»  P}-  The  flowers  are  imperfect  and  the  two  kinds  of  sporo- 


432 


THE   FAGALES 


phylls  are  usually  developed  upon  the  same  plant.  Several  bracts 
are  usually  associated  with  sporophylls  (Figs.  298,  B-F;  299, 
B-E)  so  that  the  flowers  are  of  a  higher  type  than  the  willows. 
The  innermost  of  these  bracts  is  often  of  a  delicate  structure  and 
has  been  referred  to  as  a  primitive  form  of  the  calyx  (Fig.  298, 
B,  pr)  and  when  present  in  the  pistillate  flowers  it  adheres  to 


FIG.  298.  Flowers  and  fruits  of  the  birch  family,  order  Fagales:  A,  in- 
florescence of  hornbeam  (Carpinus) — s,  staminate  ament;  p,  pistillate  ament. 
B,  staminate  clusters  from  ament  of  alder  (Alnus),  each  flower  consisting 
of  four  stamens  attached  to  a  four-parted  perianth,  pr.  C,  upper  view  of 
the  same  cluster,  showing  the  numerous  bracts  associated  with  the  flowers. 
D,  fruit  of  Carpinus  attached  f,  the  greatly  enlarged  three-lobed  bract.  D, 
fruit  cluster  of  hazel  (Corylus) — b,  bract  encircling  the  ovary  and  within  a 
more  delicate  bract,  pr,  adnate  to  the  ovary;  s,  stigma.  F,  fruit  of  Corylus, 
the  bract,  &,  in  E  has  grown  out  into  a  tubular  beaked  structure  that  com- 
pletely envelops  the  nut. 


the  ovary  (Figs.  298,  E,  pr\  300,  B,  pr).  The  pistils  are  pro- 
vided with  long  delicate  stigmas  and  are  compound,  containing 
several  ovules  but  only  one  usually  matures.  The  wall  of  the 
ovary  develops  into  a  tough  coat  about  the  seed,  forming  a  fruit 


DEVELOPMENT  OF   PLANTS  433 

known  as  the  nut.  Certain  bracts  of  the  flower  increase  greatly 
in  size  as  the  ovary  matures  and  form  a  conspicuous  part  of  the 
fruit.  Thus  in  the  hornbeam  (Fig.  298,  D),  one  of  the  bracts 
develops  into  a  large,  three-lobed  green  leaf,  in  the  hop  horn- 
beam the  bract  forms  a  papery  sac  about  the  nut,  in  the  hazel 
a  leafy  husk  (Fig.  298),  in  the  birch  and  alders  a  woody  peg- 
like  structure.  In  the  chestnut  and  beech  the  pistils  are  com- 
pletely enveloped  by  prickly  bracts  or  outgrowths  that  form  the 
bur  (Fig.  300,  A-C),  while  in  the  oak  these  structures  only  cover 
the  lower  portion  of  the  ovary,  forming  the  cup  (Fig.  299,  C-E). 
The  flowers  of  this  order  are  typical  of  those  that  are  wind  polli- 
nated. Note  the  small  and  simple  flowers,  absence  of  showy 
perianth,  nectar,  and  odor  glands,  dry  and  light  microspores 
lavishly  produced  to  ensure  seed  formation  and  the  delicate 
bushy  stigmas  for  catching  the  spores.  The  stigmas  appear  a 
day  or  so  before  the  adjoining  microspores  are  being  shed  so 
that  a  crossing  from  an  earlier  flowering  plant  is  necessitated. 
These  plants  form  the  larger  part  of  our  deciduous  forests  and 
their  association  in  colonies  is  doubtless  connected  with  the  dis- 
tribution of  pollen  by  the  wind,  as  is  also  the  appearance  of  the 
flowers  before  the  leaves  are  fully  developed.  The  positions  as- 
sumed by  the  staminate  aments  is  of  service  in  protecting  the 
microspores  against  wetting  and  also  to  assist  in  their  distribu- 
tion. During  the  winter,  these  aments  are  quite  erect  but  as  the 
flowering  stage  approaches  they  become  pendulous,  the  larger 
bracts  protecting  the  sporophylls,  like  the  shingles  on  a  roof 
(Fig.  298,  A).  These  bracts,  being  hygroscopic,  remain  closed 
during  wet  weather  but  on  dry  days  they  curve  back,  each  bract 
forming  a  shelf  which  serves  to  catch  the  spores  when  no  winds 
are  stirring  and  thus  prevent  their  falling  to  the  ground. 

140.  Other  Tree  Orders  Suggestive  of  the  Fagales.— Closely 
allied  to  the  Fagales  is  the  order  of  the  walnuts,  Juglandales, 
including  the  black  walnut  and  butternut  (Juglans)  and  the  hick- 
ories (Hicoria,  Fig.  301).  These  trees  are  of  very  common 
occurrence  in  the  northern  United  States  and  are  characterized 
by  aromatic  oils  and  large  compound  leaves.  The  flowers  are 
very  similar  in  structure  and  arrangement  to  those  of  the  beech 


434 


ORDERS   RELATED   TO   FAGALES 


order,  but  at  maturity  the  outer  part  of  the  wall  of  the  ovary 
becomes  transformed  into  a  fleshy  rind  and  the  inner  portion 
forms  a  hard  shell.  In  the  walnut  this  pulpy,  aromatic  rind' 
about  the  nut  is  finally  destroyed  by  decay,  while  in  the  hickories 


FIG.  299.  FIG.  300. 

FIG.  299.  The  Beech  family,  order  Fagales:  A ,  inflorescence  of  oak  (Quer- 
cus) — s,  staminate  ament;  p,  pistillate  inflorescence.  B,  staminate  flower 
surrounded  by  a  perianth  of  slightly  united  bracts.  C,  pistillate  flowers 
with  numerous  bracts  surrounding  base  of  ovary.  D,  section  of  flower,  the 
pistil  being  composed  of  three  carpels  and  the  inner  bracts  adnate  to  the 
ovary.  E,  fruit  of  oak,  the  cup  consisting  of  the  modified  outer  bracts  shown 
in  C  and  D  and  the  nut  has  developed  from  the  ovary  and  one  of  its  ovules. 

FIG.  300.  Flower  and  fruit  of  the  beech  (Fagus),  order  Fagales:  A,  pis- 
tillate inflorescence,  the  three-lobed  stigmas  projecting  beyond  the  bracts. 
B,  section  of  the  inflorescence — pr,  inner  bracts  or  perianth  surrounded  by 
an  outer  spiny  set.  C,  the  fruit,  the  outer  bracts  of  B  have  become  hard 
and  spiny  and  are  splitting  into  four  valves,  exposing  the  three-angled  nuts. 


the  rind  becomes  leathery  and  splits  into  valves,  freeing  the  nut 
(Fig.  301,  C). 
The  order  of  the  nettles,  Urticales,  also  has  many  points  in 


DEVELOPMENT   OF   PLANTS 


435 


common  with  the  beech  order  and  contains  several  of  our  com- 
mon trees  as  the  elm  (Ulmus,  Fig.  302),  hackberry  (Celtis),  mul- 
berry (Moms),  osage  orange  (Toxylon),  numerous  tropical 
forms,  as  the  India  rubber  trees,  banyan  tree,  etc.,  as  well  as  a 
variety  of  valuable  herbaceous  plants,  as  the  hop  and  hemp. 
These  plants  very  generally  contain  a  milky  juice,  latex,  which 


FIG.  301.  Flower  and  fruit  of  the  Juglandales:  A,  inflorescence  of  but- 
ternut— s,  staminate  ament;  p,  pistillate  inflorescence.  The  butternut  and 
black  walnut  (Juglans)  may  be  recognized  by  the  chambered  pith,  as  shown 
at  bottom  of  twig.  B,  section  of  a  pistillate  flower — pr,  perianth  adnate  to 
the  two  carpels  composing  the  pistil;  s,  stigma.  C,  fruit  of  hickory,  the 
outer  part  of  ovary  wall  splitting  into  four  valves  and  exposing  the  hard  inner 
part,  the  shell  of  the  nut. 

in  the  case  of  Ficus  and  Castilloa,  two  of  the  india  rubber  trees, 
furnishes  the  raw  material  of  india  rubber  and  in  the  cow  tree 
of  South  America  yields  a  saccharine,  nutritious  milky  juice. 
Tough  stereome  fibers  are  characteristic  features  in  many  of 
these  plants  and  furnish  the  hemp  derived  from  the  Cannabis, 
and  in  Japan  paper  is  manufactured  from  the  fibers  of  the  paper 


436 


ORDERS   HIGHER   THAN   FAGALES 


mulberry.  The  leaves  are  often  provided  with  rough  hairs  which 
sometimes  contain  irritating  acids,  as  in  the  nettles  (page  40). 
The  flowers  are  of  a  somewhat  higher  type  than  in  the  preceding 
orders,  and  associated  in  a  variety  of  dense  inflorescence.  They 
are  rarely  perfect,  though  rudimentary  (sterile)  stamens  may 
be  developed  with  fertile  pistils  or  vice  versa,  thus  giving  the 
appearance  of  perfect  flowers  (Fig.  302,  B-D).  The  calyx  does 


FIG.  302.  FIG.  303. 

FIG.  302.  Inflorescence  and  fruit  of  elm,  order  Urticales:  A,  twig  of  elm 
bearing  principally  staminate  flowers.  B,  a  staminate  flower  enlarged,  show- 
ing lobed  perianth  enclosing  numerous  stamens.  C,  pistillate  flower,  two- 
lobed  stigma  projecting  from  perianth.  D,  section  of  pistillate  flower,  show- 
ing aborted  (sterile)  stamens  and  pistil  (fertile)  of  two  carpels,  as  indicated 
by  the  two  stigmas.  E,  the  fruit  with  perianth  still  attached.  A  wing  has 
grown  out  from  the  sides  of  the  ovary. 

FIG.  303.  A  simple  type  of  the  Chenopodiales:  A,  shoot  of  Mexican  tea 
(Chenopodium) ,  showing  character  of  inflorescence,  in.  B,  flower  enlarged, 
showing  the  perianth,  stamens  and  pistil  of  three  cohering  carpels. 

not  adhere  to  the  ovary  as  in  the  Fagales,  though  the  sepals  often 
cohere  (Fig.  302,  D).  The  ovary  usually  matures  as  a  nut,  but 
in  many  cases  a  kind  of  drupe  (page  399)  is  formed,  owing  to 
the  perianth  becoming  fleshy  and  enveloping  the  nut.  The  fruit 
of  the  mulberry  is  an  aggregation  of  drupes.  In  the  fig  the  stem 
envelops  the  minute  curious  flowers  and  becomes  fleshy,  forming 
the  edible  portion  of  the  fruit. 

141.  Orders  Showing  an  Advance  over  Preceding  Forms. — 


DEVELOPMENT   OF   PLANTS 


437 


The  buckwheat  order,  Polygonales,  containing  such  familiar 
weeds  as  the  sorrel,  dock  (Rumex),  and  kno tweed  or  smartweed 
(Polygonum),  has  perfect  regular  flowers  with  a  distinct  perianth. 
These  characters  also  appear  in  the  allied  goosefoot  order,  Cheno- 
podiales,  with  its  great  array  of  common  weeds,  as  the  goosefoot 
(Cheno podium) ^  orache  (A triplex),  tumbleweed  and  pigweed 


FIG.  304.  Advanced  type  of  the  Chenopodiales:  A,  shoot  of  Melandry- 
num  bearing  flower  with  perianth  differentiated  into  calyx,  ca,  and  corolla, 
c.  B,  section  of  flower,  showing  the  relation  of  calyx  to  corolla  and  the  con- 
cealment of  the  nectar  glands  at  the  base  of  the  ovary.  Access  to  the  corolla 
tube  is  guarded  by  an  outgrowth  on  the  petals,  as  shown  in  C,  which,  assisted 
by  the  styles  or  stamens,  so  effectually  closes  the  mouth  of  the  tube  that 
only  insects  with  long  tongues  can  reach  the  honey. 

(Amaranthus) .  The  flowers  are  arranged  in  loose  clusters  and 
are  for  the  most  part  small  and  wind  pollinated  (Fig.  303) .  How- 
ever, it  is  noteworthy  that  some  of  the  genera  are  adapted  to 
insects,  the  calyx  becoming  larger  and  brightly  colored  and  nectar 
glands  are  associated  with  it.  In  the  higher  members  of  the 
order,  as  in  the  purslane  and  pink  families  (which  include  the 
spring  beauty,  Claytonia),  the  perianth  becomes  differentiated 
into  a  calyx  and  showy  corolla,  and  the  flowers  have  become  in  a 
marked  degree  adapted  to  insect  visitors  (Fig.  304).  These 


438 


THE   RANALES 


orders  form  a  natural  transition  from  the  primitive  flowers  of 
the  willows  and  beeches  to  the  large  flowers  of  the  next  order 
with  their  showy  perianths,  though  the  structure  of  the  flower 
is  not  very  indicative  of  a  relationship  between  them. 

142.  Ranales,  the  Buttercup  or  Crowfoot  Order. — This  large 
and  interesting  order  includes  a  great  variety  of  our  common 
plants,  herbs,  and  trees,  as  the  white  and  yellow  water  lilies 
(Nymphaea  and  Castalia),  buttercups  (Ranunculus),  marsh 
marigold  (Caltha),  windflower  (Anemone),  Hepatica,  rue  (Thalic- 
trum),  columbine  (Aquilegia),  larkspur  (Delphinium),  monks- 


FIG.  305.  A  common  type  of  the  Ranales,  Ranunculus  repens:  A,  habit 
of  the  plant.  B,  early  stage  of  flowering,  the  stamens  clustered  about  the 
stigmas.  C,  late  stage  of  flowering,  nearly  all  the  stamens  bent  over  towards 
the  petals,  having  discharged  their  spores.  D,  petal  with  nectar  gland  at 
base.  E,  fruit  consisting  of  numerous  spirally-arranged  akenes. 

hood  (Aconitum),  may  apple  (Podophyllum) ,  magnolia,  tulip  tree 
(Liriodendron),  Sassafras,  spice  bush  (Benzoin),  etc.  In  this 
order  we  have  again  reached  the  point,  just  as  in  the  monocoty- 
ledons (see  Liliales),  where  the  flowers  are  more  usually  solitary 
and  conspicuous,  owing  to  the  development  of  large,  showy 
perianths.  While  the  perianth  is  more  differentiated  than  in  the 
lilies  it  is  noteworthy  in  the  majority  of  the  forms  that  the  flower 
has  not  reached  the  state  where  the  calyx  is  clearly  separable  from 
the  corolla.  The  flowers  are  very  simple,  as  is  indicated  by 
the  regular  and  hypogynous  arrangement  of  the  parts  (Fig. 


DEVELOPMENT.  OF  PLANTS  439 

305).  The  receptacle  is  frequently  elongated  and  consequently 
the  organs  of  the  flower,  especially  the  sporophylls,  are  numerous 
and  indefinite  in  number  and  spirally  arranged.  Were  it  not 
for  the  perianth,  such  a  type  of  flower  would  be  quite  as  primi- 
tive as  any  of  the  preceding  orders. 

(a)  The  Buttercup,  Ranunculus. — The  flower  of  the  buttercup 
illustrates  the  more  characteristic  features  of  the  Ranales  (Fig. 
305,  A,  B).  Here  the  perianth  is  apparently  differentiated 
into  calyx  and  corolla.  The  petals,  however,  are  sometimes 
regarded  as  stamens  that  have  become  highly  modified  for  the 
production  of  nectar.  The  sporophylls  are  indefinite  in  number 
and  spirally  arranged.  It  is  interesting  to  note  the  relationship 
of  the  various  parts  of  this  simple  flower  and  their  behavior  during 
the  period  of  blooming.  When  the  flower  first  opens,  the  anthers 
are  still  closed  and  clustered  around  the  receptive  stigmas,  afford- 
ing a  natural  landing  place  for  the  insects  (Fig.  305,  B).  Thus 
crossing  may  only  be  effected  with  the  spores  carried  from  older 
flowers.  At  a  later  period  the  outermost  filaments  elongate, 
curving  over  towards  the  petals,  so  that  as  their  anthers  discharge 
there  is  little  chance  of  autogamy  (Fig.  305,  C).  This  position 
also  brings  the  anther  into  the  pathway  leading  to  the  nectar 
gland  at  the  base  of  the  petal  (Fig.  305,  /)),  so  that  the  insect 
in  probing  for  the  nectar  is  sure  to  receive  some  of  the  sticky 
microspores  on  his  body.  Each  day,  for  about  a  week,  succes- 
sive series  of  stamens  behave  in  this  manner,  and  it  is  not  until 
the  innermost  anthers  are  discharging  on  the  last  day  of  bloom- 
ing of  the  flower  that  there  is  an  opportunity  for  autogamy.  In 
many  members  of  this  and  other  orders  you  will  find  that  the 
stigmas  and  anthers  of  the  innermost  stamens  actually  meet, 
owing  to  the  peculiar  curvature  of  the  styles  and  filaments,  thus 
ensuring  autogamy  in  case  crossing  has  failed.  It  should  be 
borne  in  mind  that  the  devices  for  effecting  autogamy,  as  a  last 
resort,  are  quite  as  general  and  often  as  elaborate  as  the  pro- 
visions for  bringing  about  crossing.  The  microspores  are  pro- 
tected against  dews  and  rains  by  the  folding  of  the  perianth  and 
also  by  the  downward  curvature  of  the  pedicel  at  night,  whereas  on 
each  succeeding  bright  day  the  perianth  opens  again  and  the 


440  THE   RANALES 

flower  becomes  nearly  or  quite  erect  and  turned  towards  the 
light,  so  that  it  is  in  a  position  from  which  the  insect  will  naturally 
come.  The  opening  of  the  flower  is  due  to  a  growth  in  the  morn- 
ing of  the  basal,  inner  portion  of  the  petals  thus  bending  them 
away  from  the  center  of  the  flower  and  thus  effecting  the  opening ; 
while  a  corresponding  growth  towards  evening  of  the  basal  outer 
part  of  the  petal  bends  them  towards  the  center  of  the  flower. 
This  growth  slowly  increases  the  size  of  the  flowers,  as  may  read- 
ily be  observed  by  comparing  the  freshly  opened  flowers  of  the 
anemone,  buttercup,  etc.,  with  those  several  days  old.  All  these 
features  are  of  common  occurrence  in  a  great  variety  of  orders. 
The  fruit  of  the  buttercup  is  an  akene,  each  carpel  containing  but 
a  single  ovule  (Figs.  305,  E-,  261  B). 

(b)  Some    Variations  of  the  Order. — Many  variations  of  this 
simple  structure  appear  in  other  members  of  the  order.     The 


n 


FIG.  306.  Modifications  appearing  in  the  Ranales:  A,  flower  of  Helle- 
borus — n,  nectar  glands  due  to  modifications  of  stamens.  B,  columbine — 
w,  honey  leaves  due  to  modifications  of  the  petals  or  possibly  of  the  stamens. 

carpels  may  be  reduced  to  one,  as  in  sassafras  and  other  groups, 
and  in  the  green  hellebore  (Fig.  306,  A)  they  are  partially  com- 
pounded and  even  united  with  the  receptacle  in  some  water  lilies, 
etc.  (flowers  perigynous  and  epigynous).  The  stamens  are  some- 
times few  in  number  and  they  are  frequently  modified  into  nectar 
glands  and  assume  a  variety  of  odd  shapes  (Fig.  306,  A).  The 
showy  nectar-bearing  petals,  honey  leaves,  of  many  of  the  genera 
are  regarded  by  some  as  modified  stamens,  as  in  the  monkshood, 
columbine,  larkspur,  etc.  (Fig.  306,  B). 


DEVELOPMENT  OF   PLANTS 


441 


The  parts  of  the  perianth,  which  are  more  frequently  in  threes 
or  indefinite  in  number  than  in  fives,  are  also  subject  to  consider- 
able variation.  Very  frequently  the  corolla  is  not  developed  or 
partially  suppressed,  and  the  sepals  become  colored  and  petaloid, 
as  in  the  marsh  marigold,  hepatica,  certain  windflowers,  rue, 
spice  bush,  larkspur,  etc.  In  a  few  of  the  genera  the  perianth 
becomes  greatly  modified  and  even  irregular,  as  in  the  columbine 
(Fig.  306,  B,  n),  where  five  of  the  petals,  often  regarded  as 
modified  stamens,  are  transformed  into  tubular  honey  leaves  or 


n 


FIG.  307.  Highest  type  of  the  Ranales:  A,  inflorescence  of  monkshood. 
B,  section  of  a  flower — s,  helmet-like  sepal  enclosing  nectar  organ,  n.  C, 
flower  of  larkspur.  D,  section  of  flower — s,  spurred  sepal  enclosing  two- 
spurred  nectar  organs,  n.  E,  fruit  of  larkspur  consisting  of  3 -pistils  maturing 
as  follicles. 

into  spurs  and  hoods,  as  in  the  larkspur  and  monkshood  (Fig. 
307,  B,  D).  These  flowers  illustrate  very  well  the  progressive 
coloration  that  is  often  associated  with  the  variation  of  the  peri- 
anth. The  simpler  forms  are  usually  yellow  or  white,  while 
flowers  with  more  highly  modified  parts  are  pink  and  pale  blue 
(columbine),  blue  in  higher  types  (larkspur)  and  ultra-marine 
blue  in  the  most  irregular  and  highly  modified  type  (monks- 
hood)  .  Any  of  these  flowers  may  take  on  by  reversion  the  lower 
29 


442  THE   PAPAVERALES 

grades  of  color  so  that  a  blue-colored  form  may  vary  and  its 
offspring  may  produce  white  or  yellow  flowers,  as  in  the  colum- 
bine. The  remarkable  brilliant  yellow  of  the  flowers,  as  seen  in 
the  buttercup  and  marsh  marigold,  etc.,  has  been  regarded  as  the 
primitive  coloration  of  the  perianth,  and  it  is  to  be  noted  that 
the  flowers  showing  reversion  to  this  color  assume  a  duller 
yellow.  This  order  furnishes  a  great  variety  of  showy  orna- 
mental plants,  but  none  with  fragrant  flowers.  Many  contain 
acrid  juices  and  poisonous  alkaloids,  some  of  which  are  of  medic* 
inal  value,  as  aconite,  hydrastin,  helleborine,  etc.  Others  are  of 
value  for  the  volatile  oils,  as  sassafras,  cinnamon,  camphor,  nut- 
meg, etc.  The  simple  structure  of  the  more  characteristic  flowers 
indicates  that  this  order  is  a  very  ancient  one  and  the  pronounced 
tendency  to  vary  would  suggest  that  possibly  many  of  the  higher 
orders  of  the  Choripetalae  have  been. derived  from  this  group. 
The  origin  of  the  monocotyledons  from  this  order  has  also  been 
suggested,  owing  to  the  structural  features  of  the  embryo  and 
vascular  bundles  of  certain  groups. 

143.  Papaverales,  the  Poppy  Order. — This  group  includes  two 
very  well  known  families:  (i)  The  poppies,  with  such  familiar 
plants  as  the  poppy  (Papaver) ,  sea  poppy  (Glaucium) ,  celandine 
(Chelidonium) ,  soldier's  cap  (Bicuculla) ,  fumitory  (Adlumia), 
corydalis  (Capnoides)  (Fig.  308).  (2)  The  mustards,  includ- 
ing the  peppergrass  (Lepidium),  hedge  mustard  (Sisymbrium) , 
white  and  black  mustard  (Sinapis  and  Brassica),  yellow  rocket 
(Barbarea),  cress  (Roripa),  toothwort  (Dentaria),  shepherd's 
purse  (Bursa),  whitlow  grass  (Draba),  rock  cress  (Arabis)  (Fig. 
309).  The  families  of  the  caper  and  mignonette  are  also  in- 
cluded in  the  order.  These  plants  are  closely  related  to  the 
Ranales,  as  is  indicated  by  their  simple  and  showy  flowers,  the 
various  organs  usually  being  in  multiples  of  two  and  quite  dis- 
tinct (flowers  hypogynous)  save  in  the  case  of  the  carpels.  These 
organs  are  compound  as  in  the  water  lilies  and  more  commonly 
reduced  to  two,  thus  forming  a  sharp  contrast  with  the  preceding 
order.  Note  also  that  there  is  a  sharp  distinction  between  calyx 
and  corolla. 

(a)   The  Poppy  Family,  Papaveraceae. — The  Bloodroot,  San- 


DEVELOPMENT   OF   PLANTS 


443 


guinaria,  is  typical  of  the  simpler  members  of  this  family  (Fig. 
308).  These  plants  form  colonies  in  rather  open,  rich  woods 
owing  to  their  fleshy  rhizomes,  and  like  many  plants  with  storage 
organs,  flower  very  early  in  the  spring.  The  flowers  endure  for 
about  two  days,  but  the  large-lobed  leaves  are  conspicuous  fea- 
tures of  the  forest  carpet  for  several  weeks  while  they  are  manu- 


FIG.  308.  Examples  of  the  poppy  family,  order  Papaverales:  A,  habit 
of  the  bloodroot  at  time  of  flowering — r,  rhizome;  I,  leaf  folded  about  flower 
stalk.  B,  leaf  unfolded.  C,  section  of  flower,  showing  its  hypogynous  struc- 
ture with  numerous  stamens  and  pistil  of  two  carpels.  D,  flower  of  fumitory 
(Adlumia} — s,  sepal;  p,  inflated  petals;  w,  winged  petals  which  conceal  the 
sporophylls. 

facturing  the  food  for  the  next  season.  The  large  leaves  are 
tightly  rolled  about  the  solitary  flower  and  enveloped  in  a  papery 
sheath  as  they  emerge  from  the  soil.  A  juice,  latex,  is  found  in 
the  majority  of  the  plants  of  this  order,  and  in  this  example  it  is  of 
a  blood-red  color,  thus  giving  the  latin  and  popular  name  to  the 


444  THE   PAPAVERALES 

plant,  Sanguinaria,  or  bloodroot.  The  flowers  are  of  the  simple 
open  type  noted  in  the  buttercup,  having  two  sepals  (a  character- 
istic of  the  family)  which  fall  with  the  opening  of  the  flower, 
petals  8-12,  stamens  indefinite  and  a  compound  pistil  of  two 
carpels,  as  is  indicated  by  the  two  stigmas  and  two  rows  of  ovules 
(Fig.  308,  C).  The  stigmas  are  receptive  as  soon  as  the  flower 
opens,  but  the  anthers  remain  closed,  and  so  the  spores  must  be 
carried  from  an  older  flower.  The  petals  close  during  the  late 
afternoon  and  when  opened  on  the  following  day  the  anthers 
shed  their  spores  and  complete  the  flowering. 

In  higher  types  of  the  poppies  we  find  the  flowers  becoming 
irregular  and  the  various  parts  reduced  in  numbers.  Thus,  in 
the  soldier's  cap  (Fig.  308,  D),  there  are  two  scale-like  sepals, 
four  petals  in  two  pairs,  the  two  outer  heart-shaped  and  spurred 
at  the  base,  while  the  two  inner  are  narrow  and  winged  on  the 
back,  enclosing  the  sporophylls.  The  six  stamens  are  somewhat 
united  and  arranged  into  two  groups  opposite  the  spurred  petals. 
The  carpel  has  the  same  structure  as  in  the  bloodroot.  It  is 
evident  that  the  nectar  concealed  in  this  closed  type  of  flower 
can  only  be  secured  by  some  long-tongued  insect.  Find  out  how 
the  bee  procures  the  honey,  the  purpose  of  the  wings  on  the 
narrow  petals  and  the  position  of  the  nectar  glands.  Is  the 
stigma  mature  before  the  anthers  open?  The  structure  of  this 
higher  type  of  the  poppies  makes  the  transition  to  the  mustard 
family  a  simple  one. 

(b)  The  Mustard  Family,  Cruciferae. — Here  we  find  the 
sepals  and  petals  four  in  number,  and  the  latter  arranged  cross- 
wise (Fig.  309,  A,  B),  thus  explaining  the  family  name,  Crucif- 
erae. The  petals  usually  have  rather  long  claws  (Fig.  309,  D) 
that  are  so  associated  with  the  sepals  as  to  form  a  narrow  tubular 
perianth  that  effectually  conceals  the  nectar  glands  situated  at 
the  base  of  the  ovary.  There  are  usually  six  stamens,  but  the 
two  outer  ones  are  shorter  than  the  others  (Fig.  309,  C).  The 
ovary  is  divided  into  two  compartments  by  a  membranous  parti- 
tion. The  fruit  is  generally  a  capsule  that  usually  opens  by  two 
valves,  as  seen  in  Fig.  309,.  F.  This  large  family  of  over  1,800 
species  is  almost  universally  distributed  over  the  earth  and  of 


DEVELOPMENT   OF   PLANTS 


445 


very  common  occurrence.  These  plants  with  their  small  white 
or  yellow  flowers  aggregated  in  conspicuous  inflorescences  are 
among  the  most  familiar  weeds  of  waste  places,  fence  rows  and 
barren  fields.  The  flowers  are  of  a  more  specialized  type  than 
in  the  open  blossoms  of  the  buttercups  and  poppies,  and  present 
a  variety  of  interesting  devices  that  can  only  be  hinted  at  in  this 
lesson.  More  commonly,  perhaps,  the  stigmas  are  receptive  as 
soon  as  the  flower  opens  and  the  anthers  are  still  closed,  thus 


B 


FIG.  309.  Examples  of  the  mustard  family,  order  Papaverales:  A,  branch 
of  Brassica,  one  of  the  turnips.  B,  a  flower  enlarged.  C,  flower  in  section. 
D,  a  petal,  showing  narrow  claw  and  broad  blade.  E,  pistil.  F,  fruit,  the 
valves  of  the  two  carpels  opened  to  show  the  seeds. 


ensuring  crossing.  The  short  stamens  are  more  frequently  util- 
ized only  for  crossing  since  they  are  below  the  stigma  and  so 
placed  as  to  be  in  line  with  the  nectar  glands  that  are  often  located 
in  sac-like  enlargements  of  the  perianth.  The  four  long  stamens 


446  THE   PAPAVERALES 

are  variously  related  to  the  stigmas.  Sometimes  they  are  below 
the  stigma  and  when  open  are  of  service  only  for  crossing,  but 
later  they  elongate  and  bear  some  of  the  spores  to  the  stigma. 
The  reverse  position  of  the  sporophylls  also  occurs,  but  the  out- 
ward opening  of  the  anthers  keeps  the  microspores  from  dropping 
by  chance  on  the  stigma  until  the  stigma  is  finally  lifted  up  to 
the  anther  -by  the  elongation  of  the  style,  where  it  is  sure  to  be 
dusted  with  spores,  as  the  anthers  open  wider  and  wider.  Certain 
species  are  characterized  by  an  extraordinary  twisting  of  the 
filaments,  so  that  for  a  time  the  anther  is  turned  away  and  bent 
over  towards  the  perianth  so  as  to  be  in  line  with  the  nectar 
glands,  and  later,  at  the  close  of  the  flowering  period,  a  reversal  of 
this  bending  brings  the  anther  in  contact  with  the  stigma.  These 
illustrations  are  sufficient  to  show  that  the  flowers  have  many 
arrangements  and  exact  movements  that  result  in  crossing,  or, 
if  this  fails,  in  autogamy.  You  would  naturally  expect  that 
specialized  flowers  like  the  mustards  would  show  higher  types  of 
coloration.  This  is  indeed  the  case  in  some  forms  where  pink  and 
purple  colors  appear,  but  in  the  majority  of  the  species  the  un- 
favorable situations  in  which  they  grow  have  resulted  in  a  reduc- 
tion in  the  size  of  the  flowers  and  also  in  a  decline  in  color  to 
white  and  pale  yellow.  It  is  noteworthy  that  the  flowers  of  the 
radish  when  cultivated  on  barren  soil  develop  white,  but  upon 
rich  soil,  pink.  These  minute  flowers  have  a  decided  advantage 
in  being  grouped  in  compact  inflorescences,  a  device  that  you 
will  see  copied  again  and  again  in  the  higher  orders  because  it 
ensures  crossing  and  makes  them  conspicuous.  It  should  be 
stated  that  severalof  these  plants  get  along  very  well  without 
crossing. 

Many  products  of  great  commercial  value  are  derived  from 
various  members  of  this  order.  Opium,  from  which  morphine 
and  other  alkaloids  are  obtained,  is  the  dried  latex  obtained  from 
the  incisions  of  the  unripe  capsules  of  the  Turkish  poppy.  The 
majority  of  these  plants  contain  acrid  or  peppery  juices  that 
render  certain  parts  of  the  plant  or  their  seeds  of  value  as  spices, 
oils,  foods,  etc.  Many  of  them  are  biennial,  forming  a  close 
rosette  of  leaves  the  first  year  and  flowers  and  fruit  the  second. 


DEVELOPMENT   OF   PLANTS  447 

By  cultivation  an  abnormal  development  of  one  part  or  another 
of  the  plant  has  been  induced.  This  feature  is  well  illustrated 
in  the  cabbage,  where  the  elongated  stem  of  the  wild  plant  has 
become  shortened  by  cultivation  and  covered  with  fleshy  leaves, 
forming  a  large  bud  or  head.  Brussels  sprouts  are  a  modifica- 
tion of  the  cabbage  in  which  the  stem  becomes  more  elongated 
and  covered  with  numerous  small  heads.  The  great  variety  of 
kales  are  really  headless  cabbages  in  which  the  leaves  remain  free 
from  one  another,  assuming  a  variety  of  forms.  The  cauliflower 
is  a  variety  of  the  cabbage  in  which  the  inflorescence  has  been 
transformed  into  a  fleshy  mass  of  tissue,  and  in  the  kohlrabi  the 
stem  becomes  swollen  and  herbaceous.  The  turnip  is  a  related 
species  of  the  cabbage  genus  in  which  the  underground  portion 
of  the  plant  is  modified.  Radish,  cress,  horse-radish,  caper, 
spices  and  oils  of  mustard,  etc.,  are  other  products  of  the  order,  as 
well  as  many  cultivated  flowers,  as  the  wallflower,  stock,  mig- 
nonette, etc. 

144.  Resales,  the  Rose  Order. — This  enormous  order,  com- 
prising over  14,000  species,  is  better  known  than  any  other,  not 
only  because  of  its  great  array  of  common  field  plants,  but  espe- 
cially because  of  the  large  variety  of  our  cultivated  fruits  and 
flowers  that  belong  to  it.  The  cultivated  currant,  blackberry, 
strawberry,  apple,  pear,  cherry,  peach,  plum,  pea,  bean,  hydran- 
gea, syringa,  rose,  spiraea,  wistaria,  laburnum,  as  well  as  many 
native  trees  and  shrubs,  as  the  liquidambar,  sycamore,  witch- 
hazel,  locusts,  etc.,  belong  to  the  rose  order. 

These  plants  may  be  looked  upon  as  the  most  typical  of  the 
Choripetalae,  just  as  the  Liliales  were  the  most  representative  of 
the  monocotyledons.  The  parts  of  the  flower  are  more  com- 
monly in  fives  and  cyclic  (Fig.  310,  A)  though  the  spiral  ar- 
rangement still  persists  in  nearly  every  family  of  the  order.  The 
most  distinguishing  feature  of  the  group  is  seen  in  the  basal 
growth  of  the  receptacle,  thus  lifting  up  the  corolla  and  stamens 
about  the  ovary  (perigynous  flowers)  and  this  growth  also  fre- 
quently results  in  the  epigynous  type  of  flower.  The  simple 
forms  of  flowers  that  appear  in  many  of  the  families  are  very 
suggestive  of  the  Ranales,  the  sporophylls  being  free  and  often 


448 


THE   ROSALES 


spirally  arranged.  Thus,  in  the  live-forever  (Sedum),  we  have  a 
flower  that  has  always  been  cited  as  the  typical  flower  of  the  angi- 
osperms.  It  is  very  regular  with  five  whorls  of  alternating  parts 
of  five  members  each,  i.  e.\  5  sepals,  5  petals,  5  to  10  stamens 
(one  or  two  whorls)  and  usually  5  pistils  (Fig.  310,  A).  This  type 


D 


FIG.  310.  Simple  forms  of  the  Resales:  A,  flower  of  Sedum,  showing  the 
radial  symmetry  of  the  flower  and  five  organs  in  each  whorl.  B,  grass  of 
Parnassus  (Parnassia),  a  member  of  a  closely-allied  family.  C,  flower  on 
first  day  of  bloom,  stamens  converging  over  the  pistil  and  encircled  by  row 
of  modified  stamens.  D,  section  of  flower,  showing  slight  growth  of  base  of 
receptacle  and  consequent  adhesion  to  perianth.  This  flower  is  protandrous 
and  the  stamens  in  shedding  the  spores  straighten  up  one  by  one  on  succeed- 
ing days  and  curve  back  towards  the  petals.  The  section  shows  three  positions 
assumed  by  the  stamens.  You  may  calculate  the  significance  of  these  features. 

closely  resembles  the  arrangement  of  parts  in  the  buttercups,  save 
for  the  slight  adhesion  of  the  calyx  and  receptacle  (Fig.  310,  D). 
These  fleshy  plants  as  illustrated  in  the  houseleek  (Fig.  69),  hen 
and  chickens,  etc.,  are  very  common  forms  of  xerophytes. 


DEVELOPMENT  OF   PLANTS 


449 


Owing  to  their  ability  to  retain  moisture  (page  45)  they  can  exist 
in  the  crevices  of  rocks  and  upon  dry  soil,  while  the  formation  of 
roots  from  a  bit  of  stem  or  even  a  leaf,  as  well  as  their  habit  of 
producing  buds  on  short  stems  that  afterward  become  detached 
and  grow  into  new  plants,  brings  about  a  sure  distribution  and 
explains  their  popular  name,  live-forever. 

(a)  Some  Variations  of  the  Rose  Order. — The  active  growth 
of  the  base  of  the  receptacle  and  the  consequent  crowding  result 
in  an  interesting  series  of  departures  from  the  simple  type  noted 


FIG.  311.  Higher  forms  of  the  Resales:  A,  flower  of  the  saxifrage.  B, 
flower  in  section,  showing  the  partial  adhesion  of  receptacle  to  ovary.  C. 
inflorescence  of  currant  (Ribes).  D,  section  of  flower,  showing  receptacle 
forming  the  cavity  for  the  ovules  which  is  roofed  over  by  the  carpels,  epi- 
gynous  flower. 

above  that  lead  in  very  regular  gradations  to  the  highest  forms 
of  the  order.  In  the  higher  family  of  the  saxifrages,  for  example, 
which  includes  the  saxifrage,  false  miterwort  (Tiarella),  heu- 
chera,  bishop's  cap  (Mitella),  golden  saxifrage  (Chrysosplenium) , 
the  receptacle  generally  adheres  to  the  ovary  (Fig.  311,  A,  B). 
so  that  the  flower  is,  in  a  measure,  epigynous  and  the  carpels  are 
reduced  to  two  and  partly  fused.  Passing  to  the  families  of  the 
hydrangeas,  syringas  (Philadelphus) ,  currants  (Ribes),  etc.,  the 
receptacle  and  carpels  are  quite  fused  (Fig.  311,  C,  D)  and 


450 


THE   ROSALES 


the  flower  is  strictly  epigynous.  In  the  Rose  family,  which  is 
very  closely  connected  with  the  preceding  group,  this  story  of 
change  in  the  evolution  of  the  flower  is  repeated.  In  the  simpler 
forms  the  mass  growth  of  the  receptacle  and  calyx,  forms  a  cup- 
like  structure  (perigynous  flower).  This  cup  may  be  rathei 
broad  and  shallow  as  in  the  strawberry,  cinquefoil  and  black- 
berry (Fig.  312),  but  in  higher  types  the  receptacle  more  or  less 
completely  surrounds  the  pistils  as  in  the  spiraea,  avens,  rose, 
agrimony  (Fig.  313).  In  the  Apple  family,  including  the  apple, 


D 


FIG.  312.  FIG.  313. 

FIG.  312.  A  member  of  the  Rose  family  with  the  simple  structure  of  the 
saxifrages:  A,  flower  of  the  strawberry  (Fragaria).  B,  section  of  flower 
showing  slight  adhesion  of  receptacle  to  calyx  and  the  spiral  arrangement  of 
the  sporophylls.  C,  the  fruit,  akenes  spirally  arranged  on  the  enlarged  and 
fleshy  receptacle. 

FIG.  313.  Higher  forms  of  the  Rose  family:  C,  flower  of  Agrimonia.  D, 
section  of  flower,  showing  the  bristle-covered  receptacle  completely  surround- 
ing the  pistils. 

peach,  quince,  shadbush,  thornapple,  hawthorn,  the  ovules  are 
completely  enveloped  by  the  receptacle  (Fig.  314,  A-C)  as  in 
the  currants,  while  in  the  Plum  family,  with  its  plum,  prune, 
cherry,  peach,  almond,  apricot  members,  the  pistils  are  reduced 
to  one  and  are  quite  distinct  from  the  cup-like  receptacle  (Fig. 
3H,  D-F). 

The  Senna  family  has  essentially  the  same  type  of  flower  as 
the  plum  save  that  the  pistil  usually  contains  many  seeds  (nor- 


DEVELOPMENT   OF   PLANTS 


451 


mally  but  one  develops  in  the  plum)  and  splits  at  maturity  into 
two  valves,  a  form  of  fruit  called  a  pod.  For  example,  in  the 
honey  locust  (Gleditsia)  and  the  Kentucky  coffee  bean  (Gymno- 
cladus) ,  the  receptacle  forms  a  shallow  cup  which  bears  the  regu- 
lar sepals,  petals  and  usually  ten  stamens  about  a  single  pistil 
(Fig.  315,  A).  The  flowers  are  really  monoecious,  but  in  other 
respects  are  very  suggestive  of  the  plum  flower.  In  the  coffee 
bean  tree,  however,  the  petals  are  not  quite  equal  and  this  irre- 
gularity becomes  more  noticeable  in  the  sensitive  pea  (Cassia) 


FIG.  314.  Flowers  and  fruits  of  the  apple  and  plum  families:  A,  inflor- 
escence of  the  apple  (Mains}.  B,  section  of  flower,  showing  adhesion  of 
receptacle  to  the  ovary,  epigynous  flower.  C,  sections  of  the  fruit — c,  car- 
pels of  the  pistil;  r,  fleshy  receptacle.  D,  flower  of  cherry  (Prunus).  E, 
section  of  flower — r,  cup-like  receptacle  which  falls  off  as  fruit  matures.  F, 
fruit  in  section,  showing  the  outer  part  of  the  wall  of  the  ovary  as  a  fleshy 
rind  and  the  inner  part  forming  the  stone  or  pit  which  enclosed  a  single  seed. 


(Fig.  315,  B-D).  In  the  redbud,  or  Judas  tree  (Cercis),  the 
petals  are  very  irregular,  two  of  them  being  insecurely  united  into 
a  boat-like  structure,  known  as  the  keel,  which  encloses  the  ten 
distinct  stamens  and  single  pistil,  while  two  laterally  placed 
petals,  the  wings,  inclose  the  fifth  petal  known  as  the  standard 
(Fig.  315,  E,  F).  These  three  examples  from  the  Senna  family 


452 


THE   ROSALES 


make  a  very  easy  transition  from  the  regular  flower  of  the  plum 
to  the  highly  modified  flowers  of  the  Pea  family. 

(b)  The  Pea  Family,  Papilionaceae. — The  Pea  family  (Fig. 
316)  is  the  highest  of  the  rose  order  and  the  largest  family,  with 
one  exception,  of  all  the  angiosperms,  comprising  over  11,000 
species.  Here  we  find  the  same  type  of  flower  as  in  the  redbud. 


FIG.  315.  Development  of  the  irregular  type  of  flower  in  the  rose  order: 
A,  regular  flower  of  the  honey  locust  (Gleditsia) .  At  left  staminate  flower, 
At  right  pistillate  with  single  pistil,  c,  which  develops  into  long  flat  pod,  s. 
rudimentary  stamens.  B,  flower  of  Cassia,  showing  slightly  irregular  corolla. 
C,  section  of  flower — s,  stamen;  c,  pistil.  D,  the  fruit  or  pod.  E,  irregular 
flower  of  redbud  (Cercis) — c,  calyx;  k,  keel  enclosing  stamens  and  pistil;  w, 
wings  which  arch  over  the  standard,  s.  F,  corolla  removed,  showing  the  ten 
stamens  surrounding  the  simple  pistil,  c. 


The  standard  in  this  family,  however,  incloses  the  wings  and 
the  stamens,  usually  ten  in  number,  may  be  distinct  as  noted 
above  or  united  by  their  filaments  into  a  sheath  about  the  soli- 
tary pistil;  more  frequently  a  single  stamen  remains  free,  an 
arrangement  called  diadelphous  (two  brotherhoods).  This  type 
of  flower  is  called  papilionaceous  from  its  fancied  resemblance 


DEVELOPMENT  OF  PLANTS  453 

to  a  certain  genus  of  butterfly,  Papilio.  The  more  important 
characteristics  of  this  form  of  flower  are  very  well  illustrated 
in  the  pea  (Fig.  316).  The  standard  is  the  conspicuous  and 
most  highly-colored  organ  of  the  flower,  overlapping  the  two 
wings  which  in  turn  practically  cover  the  keel.  By  carefully 
removing  the  keel  and  wings,  it  will  be  seen  that  these  organs 
are  attached  to  the  calyx  and  receptacle  by  rather  narrow  claws 
(Fig.  316,  C)  and  that  they  are  also  locked  together  by  a  little 
process  on  each  wing  that  fits  into  a  groove  on  the  keel.  Nine 
of  the  filaments,  the  tenth  being  free,  form  a  sheath  about  the 


FIG.  316.  Structure  of  the  sweet  pea  (Lathyrus) :  A,  flower  of  the  pea 
— c,  calyx;  s,  standard  enclosing  the  two  wings,  w;  k,  keel.  B,  section  of 
flower,  showing  the  srJorophylls  concealed  in  the  keel.  C,  one  of  the  wings. 
D,  perianth  removed,  showing  relation  of  sporophylls. 

ovary  which  terminates  in  a  stylar  brush  of  upwardly  pointing 
hairs  and  in  a  stigma.  The  anthers  and  style  are  confined  in 
the  tip  of  the  keel  (Fig.  316,  B,  D).  The  significance  of  these 
features  will  be  discovered  if  you  watch  a  bee  probing  into  the 
sheath  of  filaments  after  the  nectar  secreted  at  the  base  of  the 
ovary.  The  standard  serves  to  attract  and  also  direct  his  ap- 
proach to  the  flower  so  that  he  alights  at  the  proper  place,  i.  e., 
on  the  keel  and  wings.  The  weight  of  his  body  causes  a  slight 
depression  of  the  keel  and  the  stylar  brush  sweeps  some  of  the 
microspores  upon  his  body.  It  should  be  added  that  the  flower 
is  ready  for  the  insect  as  soon  as  opened  since  the  spores  are 
discharged  before  the  flower  blooms.  Only  a  portion  of  the 
spores  are  swept  out  by  the  insect  and  the  dusting  may  be  re- 
peated on  several  visitors.  The  stigma  does  not  mature  until  a 


454  THE   ROSALES 

later  period  and  it  is  evident  that  there  is  a  good  chance  of 
autogamy  as  soon  as  the  stigma  is  mature.  Some  genera  are 
always  autogamous,  but  it  has  been  observed  in  many  cases  that 
the  spores  grow  more  vigorously  upon  the  stigma  of  another 
flower  than  upon  the  stigma  of  their  own  flower,  so  that  if  the 
stigma  receives  spores  from  its  own  and  other  flowers,  the  latter 
spores  would  develop  better  and  effect  fertilization.  This  is 
called  the  prepotency  of  foreign  spores.  In  some  species,  au- 
togamy is  impossible  owing  to  the  failure  of  the  spores  to  germi- 
nate except  when  transported  to  other  flowers.  There  was  a 
good  illustration  of  this  fact  in  Australia  when  the  English 
clover  was  introduced  and  flourished  exceedingly  well,  but  pro- 
duced no  seed  until  bees  from  England  were  introduced  for 
crossing.  Plants  transported  to  foreign  countries  are  often 
sterile  owing  to  the  absence  of  the  insect  to  which  their  varia- 
tions are  adapted.  There  are  many  variations  of  the  mechanism 
shown  in  the  pea.  In  some  genera  the  style  and  stigma  act  like 
a  piston,  pushing  out  some  of  the  spores  from  the  apex  of  the 
keel  with  each  visit  of  the  insect  and  in  some  of  these  forms  the 
wings  are  unlocked  by  the  insect  when  the  stamens  that  have  been 
retained  in  the  keel  under  considerable  pressure  are  liberated  by 
the  dropping  of  the  keel  and  spring  up,  scattering  the  spores  with 
an  explosive  effect. 

In  many  of  these  plants  the  pod  splits  at  maturity  into  two 
valves,  each  of  which  twists  with  a  sudden  snap  in  opposite  direc- 
tions, hurling  the  seeds  to  a  considerable  distance  (Fig.  317,  A). 
This  reaction  is  due  to  the  tension  set  up  by  the  difference  in 
the  drying  of  the  three  strata  of  cells  which  compose  the  walls 
of  the  pod.  The  Papilionaceae  contain  very  many  nutritious 
food  plants  as  the  pea,  bean  and  lentils,  and  also  forage  plants 
as  the  clover,  alfalfa  and  vetches.  In  temperate  regions  the 
majority  of  the  forms  are  herbaceous,  only  a  few  trees  and 
shrubs  as  the  locust  (Robinia),  broom  (Cytissus),  dye  weed 
(Genista),  false  indigo  (Amorpha)  belong  to  the  family,  but 
the  tropics  are  richly  represented  by  a  great  variety  of  woody 
forms  that  are  valuable  for  lumber,  resins  and  dyes,  as  the 
red  sandalwood,  licorice  root,  gum  tragacanth,  balsam  of  tolu, 


DEVELOPMENT  OF  PLANTS 


455 


indigo,  etc.  Related  to  the  Pea  family  are  the  curious  sensitive 
plant  (Mimosa)  and  the  acacias.  One  of  the  species  of  the 
Acacia  develops  enormous  spines  that  are  tunnelled  by  ants 
that  were  supposed  to  protect  the  trees  against  leaf-destroying 
insects.  Gum  arabic  is  obtained  from  African  and  Australian 
species  of  Acacia. 

One  of  the  most  interesting  features  of  the  Resales  is  the 
variety  of  changes  to  which  the  receptacle  and  ovaries  are  subject 
in  the  ripening  of  the  fruit.  In  the  majority  of  cases,  the  pistil 
ripens  as  an  akene  or  follicle  (splitting  along  one  side  to  free 


FIG.  317.  Fruits  of  the  Pea  family:  A,  fruit  or  pod  of  the  ground  nut, 
showing  the  manner  of  seed  dissemination  by  the  snapping  back  of  the  valves 
with  a  twisting  motion.  B,  fruit  or  lomentum  of  tick-trefoil  (Meibomia). 
The  lomentum  breaks  into  as  many  nut-like  parts  as  there  are  seeds  in  the 
fruit.  C,  hooked  bristles  on  the  surface  of  lomentum. 

the  seeds)  or  as  a  pod,  without  any  considerable  modification  in 
the  form  of  the  ovary.  In  many  instances,  however,  this  growth 
is  attended  with  pronounced  alterations  of  the  parts  as  in  the 
currants  and  gooseberries  where  the  receptacle  forms  the  ovary 
and  the  entire  structure  becomes  succulent,  forming  a  berry. 
The  same  organs  in  the  witch-hazel  develop  into  a  horn-like 
capsule,  the  walls  of  which,  after  splitting  open,  contract  and 
pinch  out  the  hard,  smooth  seed  with  great  force.  In  the  rasp- 
berries, the  ovaries  are  transformed  into  drupes  which  may  be 
lifted  off  from  the  convex  receptacle  like  a  thimble,  while  in 


456  SAPINDALES 

the  blackberries,  the  drupes  remain  attached  to  the  receptacle 
which  becomes  the  most  edible  portion  of  the  fruit.  The  at- 
tractive portion  of  the  strawberry  is  the  enlarged  succulent  re- 
ceptacle while  the  objectionable  hard  particles  on  its  surface  are 
the  akenes.  The  receptacle  surrounds  the  pistils  in  the  agrimony 
and  rose,  but  in  the  former  genus  it  is  hard  and  covered  with 
hooked  bristles  for  distribution  of  the  nut-like  fruit,  while  in  the 
rose  it  is  fleshy  and  often  brightly  colored,  serving  in  some  cases 
as  food  for  birds  and  thus  effecting  seed  distribution  (Fig.  313, 
B,  D).  The  most  pronounced  changes  are  seen  in  the  apple  and 
plum  families.  In  the  former  group,  the  carpels  become  tough, 
forming  the  "core"  (Fig.  314,  Q  and  the  edible  part  is  the 
greatly  enlarged  receptacle.  The  plum,  cherry,  peach,  etc.,  rep- 
resent the  ovary  alone,  the  cup-like  portion  of  the  receptacle  is 
cast  off  as  the  fruit  matures  and  the  ovary  becomes  a  drupe 
(Fig.  314,  F).  In  the  Pea  family,  the  ovary  usually  becomes 
a  pod  with  elastic  valves  but  it  is  also  variously  modified.  In 
the  peanut,  after  the  withering  of  the  flower,  the  pistil  is  thrust 
into  the  ground  where  it  develops  as  a  pod  that  does  not  open 
at  all.  In  other  genera,  the  pod  is  nut-like  as  in  the  clovers, 
spirally  coiled  in  alfalfa,  or  separable  into  nut-like  joints  that 
are  provided  with  hooks  as  in  the  tick  trefoil  (Meibomia)  (Fig. 
317,  B).  Doubtless  these  methods  of  distribution,  the  well- 
developed  seeds  with  their  exceptionally  firm  integuments,  the 
symbiotic  relation  of  these  plants  with  the  bacteria  (page  161) 
and  the  elaborate  mechanism  of  the  iregular  flowers  have  been 
the  causes  that  have  led  to  the  abundance  and  wide  distribution 
of  these  plants.  It  will  be  noticed  in  the  following  orders  as  in 
the  preceding  Orchidales,  that  the  irregular  and  more  highly 
constructed  flowers  are  generally  represented  by  a  great  number 
of  genera  and,  barring  some  weakness,  as  for  example,  the  poorly 
developed  embryos  of  the  orchids  and  their  peculiar  habitats,  by 
a  great  number  of  individuals  in  each  species. 

145.  Sapindales,  the  Soapberry  Order. — This  group  includes 
principally  shrubs  and  trees,  as  the  box,  sumac  (Rhus),  smoke 
tree  (Cotinus),  holly  (Ilex),  burning  bush  (Euonymus),  climb- 
ing bittersweet  (Celastrus) ,  maple  (Acer),  horse-chestnut  and 


DEVELOPMENT   OF   PLANTS 


457 


buckeye  (Aesculus),  etc.  The  flowers  are  more  commonly  small, 
of  a  yellow-green  color  and  variously  grouped  into  inflorescences 
(Fig.  318,  A).  We  noticed  in  the  rose  order  as  the  dominant 
characteristic  the  tendency  towards  the  development  of  the  basal 
region  of  the  receptacle  which  was  associated  with  the  checking 
of  its  apical  growth.  In  the  Sapindales  a  similar  shortening 
of  the  axis  without  its  basal  growth  leads  to  quite  a  different 
series  of  variations.  As  a  result  of  the  crowding  of  the  parts 
of  the  flower  upon  the  shortened  receptacle,  the  distinguishing 


FIG.  318.  A  common  form  of  the  Sapindales:  A,  inflorescence  of  sugar 
maple  (Acer  saccharuni) — st  staminate  flowers;  p,  pistillate  flowers.  B, 
staminate  (right-hand)  and  pistillate  flower  enlarged.  C,  section  of  a  pis- 
tillate flower,  showing  the  two  sterile  stamens  and  nectar  disc  at  base  of  fila- 
ments. D,  section  of  ovary,  showing  early  development  of  the  wings  of  the 
fruit  and  the  perpendicular  ovules  with  micropyle  pointing  down. 


features  of  the  order  are  seen  in  the  reduction  of  the  number 
of  the  organs  of  the  flower,  a  tendency  towards  the  cyclic  arrange- 
ment of  parts  and  an  adhesion  of  the  carpels.  For  example, 
the  sepals  and  petals  are  four  to  five  in  number,  usually  distinct 
and  the  corolla  may  be  entirely  suppressed.  There  are  two 
whorls  of  stamens,  five  to  eight  in  number,  rarely  ten,  while 
the  pistils  are  reduced  in  number,  ranging  from  two  to  more 
commonly  three  or  rarely  five.  Thus  we  see  that  the  spiral 
30 


458  THE  SAPINDALES 

type  of  flower  so  often  noticed  in  the  rose  and  preceding  orders 
has  become  reduced  as  a  rule  to  the  cyclic  type  and  that  five- 
numerous  sets  of  organs  are  not  of  common  occurrence.  This 
reduction  of  the  flower  and  the  suppression  of  parts  is  well 
illustrated  in  the  maples  (Fig.  318).  The  sepals  form  a  five- 
lobed  calyx.  The  petals  are  often  suppressed  and  a  nectar  disc 
is  developed  outside  the  stamens,  a  distinguishing  feature  of  the 
order  (Fig.  318,  C).  The  two  whorls  of  stamens  are  suppressed 
to  a  varying  degree  so  that  from  four  to  eight  commonly  appear 
and  the  pistils  are  normally  reduced  to  two.  More  frequently  the 
flowers  are  imperfect,  either  the  pistils  or  stamens  of  each 
flower  being  aborted  (Fig.  318,  B).  These  two  kinds  of  imper- 
fect flowers  are  arranged  on  the  same  or  different  trees  and  they 
are  adapted  to  small  short-tongued  lapping  insects  which  visit 
these  open  types  of  flowers.  The  intelligent  long-tongued  bees 
and  butterflies  generally  avoid  such  flowers,  having  learned  by 
experience  with  colors  and  odors  that  a  surer  supply  of  food  is 
to  be  found  in  those  flowers  that  conceal  their  nectar  and  so 
exclude  the  promiscuous  crowd  of  insects  that  swarm  about  the 
simpler  types.  It  should  be  stated  that  the  development  of 
imperfect  or  incomplete  flowers  appearing  in  many  of  the  orders 
is  not  to  be  looked  upon  as  a  primitive  condition,  though  common 
in  the  lower  monocotyledons  and  dicotyledons,  since,  throughout 
the  Angiospermae,  forms  will  constantly  appear  in  which  one  or 
another  set  of  organs  fails  to  develop.  The  formation  of  wind- 
pollinated  flowers,  however,  as  in  the  box  maple,  is  a  return  to  a 
primitive  condition. 

The  stimulation  of  fertilization  results  in  a  green  wing-like 
outgrowth  on  each  of  the  ovaries  that  assists  at  first  in  the  manu- 
facture of  food  for  the  embryo  and  later  becomes  a  dry,  mem- 
branous organ  for  seed  distribution.  This  fruit,  known  as  a 
schizocarp  or  samara,  is  at  first  partly  loosened  from  its  support 
and  remains  attached  only  by  a  small  stalk  (Fig.  319,  B),  which 
requires  a  rather  strong  wind  to  snap  it.  Thus  the  fruit  is 
freed  under  conditions  that  will  result  in  the  widest  dissemina- 
tion of  the  seed.  The  development  of  the  chlorophyll-bearing 
tissue  in  immature  fruits  to  assist  in  the  work  of  food  produc- 


DEVELOPMENT  OF  PLANTS  459 

tion  is  an  economical  arrangement  of  tissues  often  to  be  seen. 
It  is  noteworthy  that  these  green  fruits  are  often  protected  by 
bitter,  acrid  juices  and  poisonous  properties  that  finally  give  place 
to  attractive  flavors,  odors  and  colors,  variations  that  are  of 
considerable  assistance  to  seed  protection  and  distribution. 


FIG.  319.  Fruit  of  the  maple:  A,  mature  fruit  of  red  maple  (Acer  rubrum). 
B,  schizocarp  of  Norway  maple.  The  fruit  split  in  half  is  still  attached  to 
the  receptacle  by  delicate  stalks. 

Many  of  the  members  of  this  order  contain  acid  or  poisonous 
juices,  as  in  the  scarlet  fruit  of  the  sumac,  which  were  a  source 
of  acetic  acid  to  the  early  settlers  of  this  country,  or  poisonous 
oils,  as  in  the  poison  ivy  or  poison  oak  (Rhus  radicans)  and  the 
poison  sumac  (R.  Vernix).  The  former  species  is  a  climbing 
vine  or  sometimes  a  shrubby  plant  with  leaves  divided  into  three 
leaflets  and  with  nut-like  fruits  (Fig.  320,  B),  while  the  poison 
sumac  is  an  erect  coarse  shrub  ten  to  fifteen  feet  high,  with  large 
pinnate  leaves  with  reddish  petioles  and  fruit  clusters,  as  in  the 
poison  ivy  (Fig.  320,  A).  Both  of  these  plants  contain  volatile 
oils  that  cause  the  poisoning.  The  oil  may  readily  be  removed 
by  washing  in  water  containing  baking  soda,  which  saponifies  the 
oil,  or  in  alcohol  which  dissolves  it.  With  alcohol  the  washing 
must  be  thorough  in  order  not  to  spread  the  infection.  Applica- 
tions of  alcohol  containing  sugar  of  lead  (50  or  70  per  cent, 
alcohol)  are  also  recommended,  which  treatment  is  followed  by 


4<5o  GERANI  ALES— RHAM  NALES 

thorough  washing  in  alcohol.  As  in  the  preceding  orders,  the 
lower  members  of  this  group  are  characterized  by  a  regular  alter- 
nation of  the  members  of  each  whorl,  but  in  the  higher  types  the 
sets  of  organs  vary  in  number  and  the  flowers  become  irregular, 
as  in  the  horse-chestnut  and  balsam  weed  or  touch-me-not. 

146.  Orders  Suggestive  of  the  Sapindales. — The  geranium 
order,  Geraniales,  shows  essentially  the  same  type  and  range  of 
variation  in  the  flower  as  the  Sapindales,  but  there  is  this  rather 
singular  difference,  namely,  that  the  ovules,  usually  pendulous, 


FIG.  320.     Two  poisonous  species  of  the  Sapindales:  A,  Rhus  Vernix,  poison 
sumac.     B,   Rhus  radicans,   poison  ivy. 

are  always  turned  away  from  the  axis  of  the  ovary  with  micro- 
pyle  directed  upwards,  while  in  the  Sapindales  the  opposite  ar- 
rangement is  to  be  found,  micropyle  pointing  down  (Figs.  318,  D; 
321,  D).  This  order  ranges  from  the  regular  flowers  of  the 
oxalis,  flax  and  geranium  families  to  irregular  forms  like  the 
nasturtium,  milkwort  (Poly gala),  etc.  Many  of  these  plants  are 
known  by  their  peculiar  juices,  oils,  gums,  as  in  the  spurges 
(Euphorbia),  castor  bean  (Ricinus),  citron,  lemon,  orange,  etc. 
The  fruit  of  the  lemon  and  orange  is  a  berry  in  which  the  outer 
part  of  the  ovary  becomes  leathery.  The  juicy  pulp,  which  en- 
velops the  seeds,  is  formed  from  hair-like  structures  that  arise 
on  the  inner  side  of  the  carpellary  walls  and  by  degrees  entirely 
fill  them.  The  navel  orange  is  a  chance  variation  in  which  a 


DEVELOPMENT   OF   PLANTS 


461 


secondary  receptacle,  bearing  compartments  like  the  normal  fruit, 
is  introduced.  This  never  reaches  very  large  dimensions  and 
may  readily  be  seen  at  one  end  of  the  orange.  The  original 
stock  from  which  the  navel  oranges  grown  in  our  country  have 
been  derived,  was  obtained  from  Brazil. 

The  Buckthorn  order,  Rhamnales,  is  very  closely  allied  to  the 


FIG.  321.  Flowers  of  the  Geraniales  and  Rhamnales:  A,  inflorescence, 
of  Geranium.  B,  nearly  mature  fruit  consisting  of  five  united  carpels.  C, 
discharge  of  the  seed.  The  carpels  snap  apart,  owing  to  the  tension  set  up  by 
the  drying  out  of  the  tissues.  D,  section  of  pistil,  showing  the  ovules  with 
micropyle  directed  upwards.  E,  flower  of  the  grape  (Vitis),  a  common  form 
of  the  Rhamnales — ca,  calyx,  reduced  to  a  rim;  c,  corolla,  which  opens  at 
base  and  falls  off  as  a  cap;  n,  nectar  glands  within  stamens.  Compare  Sapin- 
dales.  F,  corolla  free  from  the  receptacle.  G,  flower  freed  from  corolla, 
ovary  in  section,  showing  micropyle  pointing  down  as  in  Sapindales. 

Sapindales  and  includes  such  familiar  plants  as  the  buckthorn 
(Rhamnus),  Jersey  tea  (Ceanothus) ,  used  in  revolutionary  times 
as  a  substitute  for  tea,  the  grape  (Vitis),  Japanese  ivies,  Virginia 
creeper  (Parthenocissus),  etc.  The  minute  green  or  white  flower 
shows  a  further  reduction  in  parts,  the  petals  not  only  being  fre- 
quently suppressed,  but  the  stamens  are  reduced  to  one  whorl 


462 


THE   MYRTALES 


and  placed  opposite  the  sepals,  whereas  they  alternate  with  the 
sepals  in  the  Sapindales  -(Fig.  33 1,  E-G),  and  the  nectary  is 
within  the  filaments. 

147.  Myrtales,  the  Myrtle  Order. — This  order  marks  a  decided 
advance  in  the  evolution  of  the  flower  over  preceding  groups, 
owing  to  the  mass  growth  of  the  calyx  and  receptacle  and  usually 
of  the  ovary,  the  flowers  being  perigynous  and  usually  epigynous. 
Another  important  character  is  that  the  flowers  are  always  cyclic, 
the  organs  being  regularly  arranged  in  five  whorls  of  four  or  less 


FIG.  322.  Lower  form  of  the  Myrtales,  flower  perigynous:  A,  flower  of 
Lythrum.  B,  flower  in  section — c,  lobe  of  the  tubular  calyx,  from  the  margin 
of  which  arise  the  petals,  p.  This  flower  has  the  long  form  of  style  and  the 
short  and  medium  form  of  stamens.  C,  flower  with  medium  style  and  with 
short  and  long  stamens.  D,  flower  with  short  style  and  with  medium  and 
long  stamens. 


commonly  of  five  members  each,  though  the  number  of  stamens 
may  be  greatly  increased  by  the  splitting  of  the  original  number 
of  the  set.  The  petals  are  frequently  suppressed  and  the  calyx 
is  often  highly  colored,  a  variation  to  be  seen  in  many  groups  when 
the  corolla  is  wanting.  The  carpels,  as  a  rule,  contain  numerous 
ovules  (Figs.  322,  323).  The  Myrtales  is  an  important  tropical 
order  and '  represented  with  us  by  many  familiar  plants,  such  as 
the  meadow  beauty  (Rhexia) .  willow-herb  (Epilobium) ,  the  large 


DEVELOPMENT  OF  PLANTS  463 

Evening  Primrose  family,  with  one  of  which,  Oenothera  (Fig.  323, 
A,  B),  deVries  worked  out  his  theory  of  mutation,  and  the 
common  ditch  and  pond  aquatics — the  water  milfoil  (Myrio- 
phyllum)  and  mermaid  weed  (Proserpinacd). 

The  purple  loosestrife  (Lythrum)  illustrates  the  characters  of 
the  lower  members  of  the  order  before  epigyny  has  become  estab- 
lished (Fig.  322).  The  wand-like  branches  of  this  introduced 
plant  with  their  terminal  spikes  of  purple  flowers  are  becoming 
rather  common  around  the  borders  of  marshes  and  water  ways. 
The  cup  formed  by  the  calyx  and  receptacle  bears  on  its  rim 
alternately  with  calyx  lobes  the  rather  twisted  petals  and  at  its 
base  eight  to  twelve  stamens.  The  pistil  is  composed  of  two 
carpels.  This  flower  has  become  historic  because  of  the  atten- 
tion that  has  been  given  to  its  devices  for  crossing.  Fig.  322 
shows  that  the  sporophylls  are  of  three  lengths,  short,  medium 
and  long,  an  arrangement  called  heterostyly.  It  can  readily  be 
seen  that  the  three  different  lengths  of  the  styles  correspond 
exactly  with  the  position  of  the  anthers,  consequently  whatever 
part  of  the  proboscis  or  body  of  the  bee  or  butterfly  in  visiting 
these  flowers  comes  in  contact  with  one  of  the  sets  of  open 
anthers,  exactly  the  same  region  of  their  bodies  will  touch  the 
stigmas  as  soon  as  they  visit  another  flower  with  a  corresponding 
length  of  style.  The  spores  from  the  three  lengths  of  stamens 
differ  in  size  and  color,  and  it  has  been  demonstrated  that  better 
results  follow  when  crossing  is  effected  between  sporophylls  of 
the  same  length,  i.  e.,  long  with  long,  short  with  short,  etc. 
Heterostyly  arose  in  many  of  the  orders,  as  among  some  of  the 
knotweeds,  buttercups  and  rose  families  and  in  the  gentians, 
primroses,  forget-me-not,  etc. 

A  higher  and  more  characteristic  type  of  the  order  is  seen  in 
the  Oenothera  and  in  the  great  willow-herb  (Chamaenerion,  Fig. 
323).  This  latter  plant  flourishes  in  rather  dry  soils,  forming 
large  colonies  by  its  underground  stems  and  gaining  in  con- 
spicuousness  through  its  terminal  racemes  of  large  deep  purple 
flowers.  The  parts  of  the  flower  are  in  fours.  The  receptacle 
forms  the  ovary  and  the  mass  growth  of  perianth  and  stamens 
results  in  the  development  of  a  long  tube  which  forms  at  its 


464 


THE   MYRTALES 


mouth  the  linear  segments  of  the  calyx  alternating  with  the 
round  spreading  petals  and  the  stamens.  At  the  time  of  the 
opening  of  the  flower  the  eight  anthers  are  shedding  their  spores 
and  are  in  line  with  the  nectaries,  while  the  lobed  stigma  is 
closed  and  bent  backward  (Fig.  323,  C,  s).  A  day  later  the 
stigmas  assume  the  position  shown  in  the  lower  flowers,  the  lobes 
curving  backward  so  as  to  lie  in  the  pathway  leading  to  the 
nectaries  (Fig.  323,  C,  0).  It  is  evident  that  these  positions 


FIG.  323.  Higher  forms  of  the  Myrtales,  flowers  epigynous:  A,  flower 
of  Oenothera — o,  ovary.  B,  enlarged  sectional  view  of  flower,  showing  the 
receptacle  enveloping  the  ovules.  The  receptacle  grows  above  the  ovary, 
forming  a  tube,  t,  that  bears  at  its  summit  the  lobes  of  the  calyx,  c,  the  petals, 
p,  and  stamens.  C,  inflorescence  of  the  great  willow  herb  (Chamaenerion] 
— st  closed  stigma  in  young  flowers;  0,  opened  stigmas  in  older  flowers;  a, 
stigma  touching  anthers  in  withered  flowers.  D,  capsule  opening  and  dis- 
charging the  seeds.  E,  a  seed  enlarged. — C  after  Kerner. 


must  necessitate  a  crossing  if  the  flowers  receive  the  proper  vis- 
itors. If,  for  any  reason,  crossing  should  fail  autogamy  results, 
owing  to  the  continued  curvature  of  the  stigmas,  which  are  finally 
brought  into  contact  with  the  anthers,  in  which  work  the  positions 
of  the  flowers  and  stamens  cooperate,  as  shown  in  the  lowest 


DEVELOPMENT   OF   PLANTS  465 

flower  (Fig.  323,  C,  a).  The  microspores  are  fastened  together 
in  threads  by  a  sticky  substance,  viscin,  so  that  they  adhere  to 
the  insect's  body  and  are  drawn  out  from  the  anthers  in  net-like 
skeins.  This  may  be  tested  by  applying  the  moistened  finger  to 
the  opened  anthers;  this  is  also  true  of  the  related  plants,  as  the 
fuchsia,  evening  primrose,  fireweed  (Epilobium),  enchanter's 
nightshade  (Circaea). 

The  numerous  seeds  developed  in  the  capsule  are  provided 
at  the  end  with  tufts  of  long  white  hair  (Fig.  323,  D,  £),  which 
readily  transport  them  and  explains  the  quick  appearance  of  these 
plants  in  forest  lands  that  have  been  devastated  by  fire  or  cutting . 
In  addition  to  the  native  plants  mentioned  above,  many  well- 
known  tropical  plants  are  represented  in  this  order,  as  the  man- 
grove of  our  southern  swamps  and  the  large  Myrtle  family,  often 
characterized  by  leathery  leaves  and  aromatic  oils.  This  family 
furnishes  the  clove,  which  is  a  flower  bud,  the  pomegranate, 
guava,  bay  rum,  the  Brazil  nut  and  the  eucalyptus  trees,  some  of 
which  (Australian)  attain  the  greatest  height  of  any  tree,  over 
four  hundred  feet,  with  a  diameter  of  twenty-four  feet. 

148.  Umbellales,  the  Carrot  Order. — This  order  marks  the 
consummation  in  the  reduction  and  mass  growth  of  parts  that  we 
have  seen  steadily  progressing  through  the  various  orders  of  the 
Choripetalae.  Notice  that  the  strictly  epigynous,  cyclic  flowers 
(Fig.  324,  0,  D)  are  now  reduced  to  four  whorls,  the  five  lobes 
of  the  calyx  alternating  with  the  five  petals  and  these  in  turn 
with  the  four  to  five  stamens.  The  reduction  also  appears  in  the 
pistil  which  usually  consists  of  two  carpels  that  contain  but  a 
single  ovule  each.  The  order  contains  two  rather  small  families, 
the  Ginseng  family  (Araliaceae)  represented  by  the  sarsaparilla 
or  spikenard  (Aralia),  the  ginseng  (Panax)  and  the  Dogwood 
family  (Cornaceae),  including  the  pepperidge  or  sour  gum 
trees  (Nyssa),  dogwood  or  cornel  (Cornus)-,  and  the  large  Carrot 
family  (Umbelliferae)  of  2,100  species.  The  Carrot  family 
contains  many  familiar  native  and  cultivated  plants  which  may 
be  recognized  by  the  hollow  internodes  of  the  stems,  leaves  vari- 
ously lobed  and  attached  by  conspicuous  sheathing  petioles, 
peculiar  odors  derived  from  oils  and  resins,  small  flowers  that  are 


466 


THE   UMBELLALES 


usually  white  or  yellow  and  grouped  into  flat-topped  inflores- 
cences (umbels),  which  are  usually  surrounded  by  bracts,  called 
the  involucre  (Fig.  324,  A,  B).  These  features  are  well  seen  in 
the  wild  carrot  which  has  become  a  troublesome  weed  in  pasture 
lands  and  meadows.  The  calyx  lobes  are  very  small,  a  feature 
likely  to  be  seen  in  any  epigynous  flower.  The  incurved  petals 
alternate  with  the  five  stamens  and  in  the  center  of  the  flower,  the 


FIG.  324.  A  common  form  of  the  Umbellales:  A,  stems  of  wild  carrot 
(Daucus}  with  flowers  arranged  in  compound  umbels.  B,  inflorescence  with 
all  the  umbels  removed  but  one — in,  bracts  of  the  involucre.  C,  forms  of 
flowers,  the  one  on  the  right  being  a  flower  from  the  margin  of  the  umbel. 
The  corolla  is  enlarged  and  irregular,  thus  adding  to  the  conspicuousness 
of  the  inflorescence.  D,  corolla  partially  removed  to  show  the  epigynous 
character  of  the  flower  and  the  cushion-like  nectary  at  the  base  of  the  styles ; 
o,  ovary.  £,  fruit  splitting  into  two  nutlets.  F,  portion  of  hollow  stem 
with  sheathing  of  leaf  base. 


two  styles  broaden  out  into  a  conspicuous  nectary  above  the 
ovaries.  The  fruit  is  a  schizocarp,  splitting  into  two  nut-like 
parts  after  the  manner  of  the  maple  (Fig.  324,  E).  The  flowers 


DEVELOPMENT  OF   PLANTS  467 

on  the  outside  of  the  umbel  become  somewhat  irregular,  owing  to 
the  unequal  enlargement  of  the  outermost  petals.  These  small 
flowers  that  individually  cannot  be  seen  at  a  distance  of  a  few  feet 
become  the  most  conspicuous  feature  of  our  fields  owing  to  their 
association  in  large  numbers  into  compact,  lace-like  umbels.  The 
aggregation  of  flowers  has  been  noticed  in  various  groups,  as  in 
the  mustards,  various  families  of  the  Resales,  in  the  Sapindales 
and  buckthorns,  of  which  latter  simple  regular  type  the  carrot 
flower  would  appear  to  be  a  natural  sequence  (compare  Figs. 
321,  G;  324,  D).  But  in  no  group  has  so  successful  and  varied 
an  arrangement  of  the  flowers  been  achieved.  The  advantages 
of  this  umbellate  arrangement  are  apparent.  The  crowding  of 
the  flowers  is  attended  with  the  reduction  in  their  size  and  in 
saving  of  material  while  they  have  gained  in  the  number  of 
flowers  and  in  conspicuousness.  This  arrangement  also  increases 
the  chances  of  seed  production  since  the  insect  in  a  single  visit 
may  cross  a  score  of  flowers  in  crawling  over  the  umbel.  Autog- 
amy is  prevented  at  first  by  the  difference  in  the  maturation  of 
the  anthers  and  stigmas,  frequently  one  or  more  days  interven- 
ing between  these  two  conditions ;  and  in  many  genera  the  flowers 
are  imperfect,  the  stamens  and  pistils  being  located  on  differ- 
ent parts  of  the  umbel  and  so  further  assist  in  crossing.  But 
even  if  the  great  variety  of  small  lapping  insects  that  swarm  over 
these  flowers  were  absent  the  construction  of  the  umbel  is  such 
that  the  flowers  are  usually  able  to  effect  pollination  unassisted. 
The  variety  of  devices  for  the  accomplishment  of  this  work  is 
without  parallel  in  any  other  group.  The  opening  of  the  flowers 
of  an  umbel  proceeds  either  from  the  circumference  towards 
the  center  or  from  the  center  outwards.  In  some  genera,  as 
Eryngium,  the  flowers  on  the  margin  open  first  and  the  stigmas 
are  ready  for  crossing  while  the  anthers  are  closed  and  bent 
down  upon  the  petals.  On  the  following  day,  the  inner  adjoining 
set  of  flowers  is  in  the  same  condition  while  the  anthers  of  the 
marginal  flowers  have  been  lifted  up  by  their  filaments  so  that 
they  reach  out  and  come  in  contact  with  the  stigmas  of  the  ad- 
jacent inner  flowers,  the  curvature  of  the  styles  often  assisting  in 
bringing  the  two  organs  together.  In  this  way,  ample  oppor- 


468  THE   UMBELLALES 

tunity  for  crossing  with  the  older  flowers  of  another  umbel  is 
first  given  and  later  a  crossing  between  the  adjacent  flowers  of 
the  same  umbel  is  almost  sure  to  result.  In  other  cases,  as  the 
snakeroot  (Sanicula)  a  few  of  the  simple  flowers  are  perfect 
while  the  remaining  flowers  of  the  umbel  bear  only  stamens. 
The  perfect  flowers  are  the  first  to  open,  being  in  the  same  con- 
dition as  the  marginal  flowers  of  the  preceding  example.  Later 
the  filaments  straighten  out,  curve  away  from  the  stigmas  and 
the  anthers  discharge  their  spores  and  finally  drop  off.  Now  the 
adjoining  flowers  open  and  extend  their  filaments  so  that  the 
anthers  are  pushed  over  to  the  stigmas  of  the  perfect  flowers.  In 
the  chervil,  a  similar  crossing  is  effected  after  the  perfect  flowers 
have  shed  their  anthers  by  the  numerous  imperfect  flowers  grow- 
ing above  them  and  extending  their  anthers  so  that  the  micro- 
spores  will  drop  straight  down  upon  the  stigmas  (Fig.  325). 


FIG.  325.  Flowers  of  chervil:  A,  perfect  flowers  only  in  bloom.  The 
anthers  are  discharging  their  spores  before  the  stigmas  are  receptive.  B, 
later  stage — the  staminate  flowers  are  now  in  bloom  and  shedding  their  spores 
upon  the  mature  stigmas  of  the  perfect  flowers. — After  Kerner. 

In  the  water  parsnip  (Slum),  and  beaked  parsley  (Anthriscus) , 
etc.,  there  are  two  kinds  of  umbels,  one  containing  principally 
perfect  flowers  and  the  other  only  staminate  flowers.  The  per- 
fect flowers  blossom  first,  and  shed  their  spores  and  anthers  be- 
fore the  stigmas  are  receptive.  After  all  the  anthers  have  been 
shed,  the  stigmas  mature  and  continue  in  this  condition  for  two 
days,  during  which  time  the  umbels  of  imperfect  flowers  have 
grown  above  them  and  assume  such  a  position  that  when  the 
anthers  finally  open  there  results  a  rain  of  microspores  upon  the 
stigmas  below. 

The  members  of  the  Carrot  family  are  characterized  by  the 


DEVELOPMENT  OF   PLANTS  469 

presence  of  essential  oils  and  resins  in  the  roots,  stems,  leaves  or 
especially  in  the  fruits  which  usually  contain  oil  cavities  between 
the  ribs  of  the  ovary.  These  substances  are  the  sources  of  the 
peculiar  odors  that  make  easy  the  recognition  of  these  plants. 
These  oils  give  the  commercial  value  to  several  fruits,  as  in  the 
caraway,  parsley,  fennel,  anise,  coriander.  Several  species  con- 
tain very  poisonous  alkaloids,  as  the  water  hemlock  (Cicuta), 
among  the  most  poisonous  of  our  native  plants,  and  the  hemlock 
(Conium)  which  perhaps  furnished  the  potion  drunk  by  Socrates. 
Several  valuable  food  plants,  as  the  carrot,  parsnip,  celery,  par- 
sley, are  cultivated  species  of  the  family. 

Series  b.     Sympetalae. 

149.  General  Characteristics. — This  group  contains  the  most 
common  and  abundant  of  our  flowering  plants,  approximating 
42,000  species,  and  ranks  as  the  most  specialized  of  the  angio- 
sperms.  The  variations  of  these  plants  have  been  very  success- 
ful, especially  to  be  noticed  are  the  development  of  underground 
stems,  special  types  of  flower  structures  and  the  massing  of  the 
flowers  in  dense  inflorescences.  As  a  result  of  these  advantages, 
they  exceed  all  other  groups  in  the  number  of  individuals,  though 
they  comprise  fewer  orders  as  a  consequence  of  the  striking 
uniformity  in  the  floral  structure. 

While  a  few  of  the  simpler  members  of  this  group  have  dis- 
tinct petals,  the  crowding  of  the  organs  on  the  receptacle  has 
resulted,  in  nearly  all  the  forms,  in  the  sympetalous  type  of 
corolla,  in  the  cyclic  arrangement  of  the  parts,  and  in  the  reduc- 
tion in  the  number  of  stamens,  so  that  each  whorl  does  not  exceed 
the  number  of  petals,  and  more  frequently  one  whorl  of  stamens 
is  partly  or  entirely  suppressed.  The  pistils  are  usually  less 
numerous  than  the  petals.  The  flowers  range  from  regular 
hypogynous  forms  to  epigynous  and  irregular  types  through  the 
same  series  of  variations,  as  noted  in  the  Choripetalae.  The 
Sympetalae  are  the  most  recently  evolved  of  the  Angiospermae, 
as  is  apparent  from  their  uniformity  of  structure  and  they 
appear  to  be  adapted  to  temperate  and  northern  conditions  where 
they  have  preempted  the  open  country  and  flourished  exceedingly, 


470 


ERICALES 


forming  the  most  characteristic  features  of  the  herbaceous  flora. 
Aquatics  are  of  rare  occurrence  and  but  few  tree  forms  appear; 
notably  the  ash,  persimmon,  catalpa,  paulownia,  etc.  Heath - 
like  shrubs,  however,  are  widely  distributed  in  northern  regions. 
The  invasion  of  the  tropics  with  its  favorable  conditions  has  led 
to  an  enormous  increase  in  some  of  the  groups  and  the  develop- 
ment also  of  a  great  variety  of  woody  plants,  as  trees  and  climbers. 
150.  Ericales,  the  Heath  Order. — This  order  is  the  simplest  of 
the  Sympetalae  and  includes  a  great  variety  of  plants  that  are 
largely  northern  in  their  distribution.  Many  are  cultivated,  as 
the  azaleas,  rhododendrons  and  laurels  (Kalmia),  and  others  are 
familiar  plants  of  bogs  and  woods  as  the  sweet-pepper  bush 


FIG.  326.  A  simple  form  of  the  Ericales:  A,  the  shin  leaf  (Pyrola)  with 
flower  just  opened.  B,  flower  with  part  of  perianth  removed  to  show  the 
retention  of  the  anthers  by  the  petals.  C,  later  stage  of  flowering.  The 
anthers  have  been  released  by  the  spreading  of  the  petals  and  the  flower 
stalk  has  inclined  so  that  the  spores  sprinkle  down  on  the  stigma.  D,  flower 
in  section,  showing  the  relation  of  parts  in  autogamy. — After  Kerner. 

(Clethra),  wintergreen  (Chimaphila) ,  Labrador  tea  (Ledum), 
Rhodora,  Leucothoe,  rosemary  (Andromeda),  leather  leaf  (Cham- 
aedaphne),  arbutus  (Epigaea),  checkerberry  (Gaultheria),  bear- 
berry  (Arctostaphylos) ,  huckleberry  (Gaylussacia) ,  blueberry 
(Vaccinium),  heather  (Calluna),  cranberry  (Oxycoccus).  The 
flowers  of  the  heaths  are  very  characteristic  and  sharply  dis- 


DEVELOPMENT   OF   PLANTS  471 

tinguished  from  other  orders.  More  frequently  they  consist  of 
five  regular  whorls  of  five  members  each,  anthers  usually  opening 
by  pores  and  often  provided  with  two  horn-like  appendages,  and 
the  pistil  is  composed  of  five  carpels,  the  ovary  maturing  as  a 
capsule  or  berry  (Fig.  326).  The  separate  origin  of  each  whorl  of 
organs  is  an  important  feature  of  the  order  as  contrasted  with 
other  Sympetalae,  each  set  being  as  a  rule  separately  attached  to 
the  receptacle.  While  the  sympetalous  corolla  is  a  step  in  ad- 
vance of  previous  types,  it  is  evident  that  these  regular  hypogy- 
nous  flowers  are  in  other  respects  of  a  rather  simple  character. 
This  feature  is  further  illustrated  in  some  of  the  genera  like 
Clethra,  Monotropa  and  Pyrola  where  the  free  or  slightly  fused 
petals  form  a  very  natural  transition  from  the  Choripetalae 
(Figs.  327,  A\  326). 

The  simpler  forms  of  flowers  are  illustrated  in  Pyrola  with 
its  regular  hypogynous  and  polypetalous  flowers,  though  some 
species  show  a  slight  tendency  to  sympetaly.  The  stigmas  are 
mature  with  the  opening  of  the  flower  while  the  anthers  are 
bent  back  out  of  the  way,  their  filaments  being  held  under  con- 
siderable tension  by  the  petals  (Fig.  326,  A,  B).  A  bee  laden 
with  spores  would  effect  a  crossing  as  soon  as  he  alighted  upon 
the  flower,  but  in  probing  after  the  nectar  he  presses  back  the 
petals,  thus  releasing  the  anthers  which  snap  down,  spilling  the 
spores  upon  him.  Doubtless  autogamy  results  in  many  of  tHese 
forms  if  crossing  fails  owing  to  the  expansion  of  the  petals 
which  would  release  the  stamens,  and  the  drooping  of  the  flower 
which  would  bring  the  falling  spores  in  line  with  the  stigma 
(Fig.' 326,  C,  D). 

Other  genera  show  varying  degrees  of  sympetaly  but  in  the 
majority  of  cases,  tubular  corollas  appear  (Fig.  327,  C)  as  in  the 
Leucothoe,  bearberry,  Andromeda,  blueberries,  etc.  The  majority 
of  such  forms  have  horned  anthers  which  are  shaken  by  the 
insect  and  so  assist  in  sifting  out  the  spores.  In  the  Blueberry 
family,  we  note  that  a  mass  growth  of  the  receptacle  and  ovary 
has  begun  (Fig.  327,  D)  while  in  Rhodora,  rhododendron  and 
azaleas  another  line  of  variation  appears  in  the  irregular  corolla 
(Fig.  327,  F).  In  the  two  latter  genera,  the  styles  and  stamens 


472 


THE   ERICALES 


extend  far  out,  furnishing  a  natural  landing  place  for  hovering 
animals  as  moths  and  humming  birds.  In  this  position  the 
spores  cannot  be  sifted  out  and  we  find  them  fastened  together 
in  fours,  just  as  they  were  formed  in  the  mother  cell,  by  sticky 
threads  which  cause  them  to  adhere  on  the  insect's  body  in 
fringe-like  masses  (Fig.  327,  G).  One  of  the  lower  basidiomy- 
cetes  (Exobasidium)  infests  the  leaves  and  flowers  of  the  azalea 
and  other  genera,  causing  large  watery  outgrowths  that  are 


FIG.  327.  Common  forms  of  Ericales:  A,  Indian  pipe  (Monotropo). 
Leaves  reduced  to  scales  and  without  chlorophyll,  owing  to  saprophytic 
habit  of  plant.  B,  section  of  flowers,  showing  polypetalous  corolla.  C, 
inflorescence  of  the  blueberry  (Vaccinium).  D,  section  of  flower,  showing 
sympetalous  corolla  and  epigynous  type  of  flower.  E,  a  stamen  enlarged, 
opening  by  pores  at  end  of  anther.  F,  Azalea,  showing  irregular  type  of  flower. 
G,  spores  attached  by  viscid  threads. 

often  eaten  under  the  impression  that  they  are  the  fruit  of  the 
plant.  The  members  of  this  order  are  adapted  to  a  wide  range 
of  conditions  though  more  characteristic  of  north  temperate  and 


DEVELOPMENT   OF   PLANTS  473 

arctic  regions  where  they  constitute  the  conspicuous  features  of 
the  tundras,  bogs,  heaths  and  moors.  These  plants  are  often 
characterized  by  their  thick,  leathery,  evergreen  leaves  of  pro- 
nounced xerophytic  character  and  their  common  distribution 
in  wet  moors  and  bogs  presents  a  most  puzzling  problem  in  the 
association  of  plants.  Many  of  the  heaths  have  been  cultivated 
from  very  early  times  and  they  have  lent  themselves  so  readily 
to  crossing  that  perhaps  no  order  furnishes  so  many  forms  for 
decorative  and  landscape  effects.  Several  yield  valuable  fruits, 
as  the  blueberries,  huckleberries  and  cranberries. 

151.  Orders  of  a  Higher  Type  than  the  Ericales.— The  prim- 
rose order  (Primulales)  forms  a  very  natural  transition  from  the 
Ericales.  As  the  next  step  in  advance,  we  note  the  slight  associ- 
ation of  the  stamens  and  the  corolla  (Fig.  328,  A)  and  their 
frequent  reduction  in  number.  The  occurrence  of  staminodia, 
page  421,  in  certain  forms  marks  the  first  transition  to  the 
reduction  of  the  number  of  stamens.  The  pistil  is  generally 
composed  of  five  carpels  forming  a  capsule  without  partitions 
(unilocular)  and  containing  a  central  placenta,  bearing  numerous 
seeds,  that  is  attached  only  to  the  base  of  the  ovary  (Fig.  328,  A). 
This  is  the  so-called  free  central  placenta  and  only  occurs  else- 
where in  the  small  Bladderwort  family  of  aquatics.  This  order 
includes  the  loosestrife  (Lysimachia  and  Steironema) ,  star  flower 
(Trientalis) ,  shooting  star  (Dodecatheori)  and  cultivated  forms  of 
the  cyclamen  and  primroses. 

Proceeding  to  the  gentian  order  (Gentianales) ,  it  will  be 
noted  that  the  flowers  are  variable  and  not  as  clearly  character- 
ized, the  corolla  being  sometimes  wanting  or  polypetalous.  The 
reduction  of  the  stamens  to  a  single  whorl  and  the  pistils  usually 
to  two  in  number  indicates  points  of  advance  that  are  to  be 
associated  with  the  twisting  of  the  petals  in  the  bud  and  the 
opposite  arrangement  of  the  leaves  as  distinguishing  features 
of  the  order  (Fig.  328,  B-D).  Note  should  be  made  of  the  fact 
that  the  two  pistils  are  often  quite  distinct,  doubtless  a  survival 
of  a  more  primitive  condition.  The  Gentianales  include  the  ash 
(Fraxinus),  lilac  (Syringa),  Forsythia,  olive  (Olea),  valued  foi 
fruit  and  oil,  fringe  tree  (Chionanthus),  privet  (Ligustrum),  jas- 
31 


474 


THE   POLEMONIALES 


mine,  Strychnos,  yielding  the  alkaloid  strychnine,  oleander 
(Nerium),  the  gentians,  marsh  pink  (Sabbatia) ,  the  aquatic  float- 
ing heart  (Limnanthemum)  and  the  highly  specialized  plants  of 
the  dogbane  and  milkweed  families.  The  latter  family  is  dis- 
tinguished by  the  microspores  being  united  into  club-shaped 
pollinia  that  are  attached,  one  from  each  of  the  two  adjoining 
anthers,  to  a  peculiar  hook,  so  that  the  leg  of  the  insect  becomes 
caught  in  the  hook  and  withdraws  the  pollinia  in  pairs. 


FIG.  328.  Primulales  and  Gentianales:  A,  flower  of  loosestrife  (Steiro- 
nema).  Above  flower  in  section,  showing  cohesion  of  single  row  of  stamens 
to  base  of  corolla  and  the  numerous  ovules  on  a  free  central  placenta.  These 
are  the  most  important  characteristics  of  the  order.  B,  fringed  gentian 
(Gentiana) ,  showing  leaf  arrangement  and  twisting  of  petals  in  bud,  b,  char- 
acteristics of  the  order.  C,  section  of  flower.  D,  fruit,  the  two  carpels 
separated. 

152.  Polemoniales,  or  Phlox  Order. — This  group  is  the  richest 
in  the  number  of  species,  over  14,600,  of  any  order  of  angio- 


DEVELOPMENT   OF   PLANTS  475 

sperms.  The  advance  over  previous  orders  is  seen  in  the  more 
prolonged  mass  growth  of  the  corolla  and  stamens,  the  filaments 
appearing  to  rise  at  a  higher  point  on  the  corolla  (Fig.  330,  D) 
and  the  stamens  are  reduced  to  a  single  whorl,  and  frequently  less 
than  five  in  number.  The  pistils  are  completely  compound  and 
usually  composed  of  but  two  carpels.  This  reduction  is  asso- 


FIG.  329.  FIG.  330. 

FIG.  329.  Flower  of  the  morning-glory  (Ipomoea),  showing  the  tubular 
corolla  characteristic  of  the  Polemoniales. 

FIG.  330.  Boraginaceae:  C,  inflorescence  of  comfrey  (Symphytum) .  Note 
the  coiled  inflorescence,  a,  a  feature  of  this  family.  D,  section  of  flower, 
showing  the  deeply  four-lobed  ovary  and  the  stamens  cohering  high  on  the 
corolla  and  alternating  with  small  tongue-like  scales. 

ciated  with  a  pronounced  irregularity  of  the  corolla  in  the  higher 
families  and  a  high  degree  of  specialization  in  the  construction 
of  the  flower  which  in  part  accounts  for  the  occurrence  of  the  large 
number  of  individuals.  The  order  is  noted  for  its  great  number 
of  showy  flowers  and  the  large  tubular  corollas  which  attain  their 
highest  perfection  in  several  of  the  families. 

(a)  The  More  Important  Families  of  the  Phlox  Order. — The 
flowers  of  the  lower  families  are  regular,  as  in  the  morning- 
glory,  sweet  potato,  dodder  (Cuscuta),  a  yellow  thread-like  para- 


476  THE   POLEMONIALES 

site  that  twines  about  various  plants,  and  the  phlox  (Fig.  329). 
The  flowers  of  the  rough-leaved  borages  (Boraginaceae)  are 
also  regular  but  they  are  distinguished  by  having  the  ovary  of 
the  two  carpels  deeply  lobed  so  that  the  fruit  appears  as  four 
nutlets  and  also  by  their  coiled  inflorescences  as  shown  in  Fig. 
330.  This  family  includes  the  heliotrope,  hound 's-tongue  (Cyno- 
glossum),  forget-me-not  (Myosotis),  comfrey  (Symphytum) ,  and 
the  blueweed  (Echium)  in  which  the  corolla  becomes  irregular. 
The  Mint  family,  Labiatae,  is  world-wide  in  its  distribution 
and  the  largest  of  the  order  with  3,000  species.  These  plants 
are  sharply  characterized  by  square  stems,  opposite  leaves,  aro- 
matic oils,  nut-like  fruits  as  in  the  borages,  but  the  corolla  is 
very  irregular  and  usually  two-lipped.  The  stamens,  usually 
two  short  and  two  long  ones,  together  with  the  two  lobed  style 
are  more  frequently  concealed  under  the  upper  lip  of  the  corolla 
(Fig.  331).  The  flowers  are  generally  so  placed  that  the  lower 
lip  serves  as  a  landing  place  for  insects  and  in  entering  the 
flower,  the  top  of  their  bodies  rubs  against  the  stigmas  or  anthers. 
Crossing  is  secured  by  the  difference  in  the  time  of  the  matura- 
tion of  the  anthers  and  stigmas,  assisted  by  a  variety  of  move- 
ments, such  as  the  alternate  curving  down  of  the  anthers  and 
stigmas  which  brings  first  one  set  of  organs  and  then  another 
into  the  pathway  of  the  insect.  One  form  of  this  arrangement 
is  seen  in  the  sage,  where  there  are  but  two  strangely  modified 
stamens,  the  two  anther  lobes  being  connected  by  a  long-curved 
rod  which  is  fastened  near  one  end  to  a  short  filament  so  that 
the  anther  becomes  a  lever  with  the  filament  as  a  fulcrum  (Fig. 
332).  Only  the  anther  lobes  concealed  under  the  upper  lip  are 
fertile.  As  the  bee  enters  the  flower,  he  pushes  up  the  short 
arm  of  the  lever,  thus  causing  the  long  arm  to  swing  down, 
pressing  the  fertile  anther  upon  his  back  and  dusting  him  with 
spores.  Later,  the  style  which  is  at  first  immature  and  above 
this  apparatus,  grows  out  and  curves  down  and  at  maturity 
comes  to  lie  in  such  a  position  as  to  rub  upon  the  back  of  a 
visiting  insect  at  the  same  point  as  the  anthers.  The  peculiar 
forms  of  the  corolla,  the  relation  of  the  anthers  and  stigmas  and 
the  frequent  occurrence  of  hairs  and  bristles  that  direct  the  in- 


DEVELOPMENT   OF  PLANTS 


477 


FIG.  331.  FIG.  332. 

FIG.  331.  A  common  species  of  the  Mint  family:  A,  inflorescence  of  the 
skullcap  (Scutellaria).  Note  the  square  stem,  opposite  leaves.  Why  are 
all  the  flowers  facing  one  way?  B,  flower  enlarged,  showing  the  two-lobed 
under  lip  and  the  three-lobed  upper  lip  which  conceals  the  sporophylls.  C, 
section  of  the  flower.  Ovary  four-lobed,  stamens  cohering  with  the  corolla 
and  anthers  concealed  with  the  stigma  beneath  upper  lip.  Purpose  of  the 
crest,  c,  on  the  calyx? 

FIG.  332.  Flower  of  the  sage  (Salvia):  A,  flower  after  the  anthers  have 
shed  their  spores.  The  two-lobed  stigma  is  bending  down  into  the  position 
occupied  by  them.  B,  sectional  view  of  the  flower,  showing  four-lobed  ovary 
with  nectar  glands  at  the  base,  stigma  not  receptive  and  bent  back.  As  the 
insect  enters  the  flower  he  pushes  against  the  sterile  lobe  of  the  anther,  /,  and 
thus  causes  the  fertile  lobe,  a,  to  swing  down  upon  his  back.  /,  filament  of 
anther. 


478  THE   POLEMONIALES 

sect  with  the  greatest  precision  to  particular  parts  of  the  flower 
with  a  view  to  crossing,  mark  this  family  of  the  mints  as  the 
most  specialized  of  the  order.  A  fine  series  of  devices  are  seen 
in  such  common  mints  as  the  blue  curls  ( Trichostema) ,  skull- 
cap (Scutellaria) ,  hyssop  (Agastache),  ground  ivy  (Glecoma), 
catmint  (Nepeta),  selfheal  (Prunella),  motherwort  (Leonurus), 
dead  nettle  (Lamium),  hedge  nettle  (Stachys),  sage  (Salvia), 
bergamot  (Monarda),  pennyroyal  (Hedeoma),  mountain  mint 
(Koellia),  bugle  weed  (Lycopus),  etc.  Many  of  these  plants 
are  of  commercial  importance.  Mentha  yields  valuable  oils  as 
spearmint,  peppermint,  and  menthol  and  from  other  genera  are 
derived  oils  used  in  perfumery,  medicine  and  as  condiments, 
as  rosemary,  lavender,  origanum,  thyme.  Several  are  cultivated 
for  their  flowers  and  foliage,  as  the  coleus,  monarda,  stachys,  etc. 
The  large  family  of  figworts,  Scrophulariaceae,  with  its  2,400 
species,  closely  resembles  the  mints,  but  is  distinguished  by  the 
ovary  being  undivided  and  containing  many  seeds  on  a  central 
axis  (Fig.  333,  C).  While  the  simpler  forms  have  almost  regular 
corollas  and  five  stamens,  the  higher  forms  are  bilabiate  with 
two  to  four  stamens  as  in  the  mints.  This  family  contains  some 
of  the  most  showy  of  our  cultivated  plants,  as  the  foxglove 
(Dasystoma  and  Digitalis),  Gerardia,  snapdragon  (Antirrhi- 
num), rattlebox  (Rhinanthus) ,  toadflax  (Linaria),  Paulownia, 
beardtongue  (Pentstemori),  monkey  flower  (Mimulus);  and  the 
allied  Catalpa  and  trumpet  creeper;  also  many  other  attractive 
plants  that  are  not  cultivated  as  the  mullen  (Verbascum),  fig- 
wort  (Scrophularia) ,  turtle  head  (Chelone),  hedge  hyssop  (Gra- 
tiola),  speedwell  (Veronica),  painted  cup  (Castill'eja) ,  lousewort 
(Pedicularis) ,  cow- wheat  (M elampyrum) .  These  plants  are 
among  the  most  characteristic  features  of  our  flora,  the  family 
being  largely  confined  in  its  distribution  to  the  north  temperate 
regions.  The  flower  of  the  toadflax  (Linaria)  illustrates  a  com- 
mon type  of  the  Figwort  family  (Fig.  333).  Such  flowers  are 
said  to  be  personate,  that  is  bilabiate  but  with  the  under  lip 
arched  so  as  to  meet  the  upper  lip  and  entirely  closing  the  mouth 
of  the  corolla.  This  arrangement  very  effectively  protects  the 
microspores  and  conceals  the  nectar  and  it  requires  a  rather 


DEVELOPMENT  OF   PLANTS 


479 


muscular  insect  alighting  upon  the  knobbed  lower  lip  to  force 
his  way  into  the  flower.  It  is  worth  any  one's  time  to  sit  by 
this  plant  and  examine  the  mechanism  of  the  flower  while  the 
bee  is  at  work.  The  sporophylls  present  the  same  variety  of 
arrangements  for  crossing  as  noted  in  the  mints.  The  nectar 
glands  are  situated  at  the  base  of  the  carpels  but  the  nectar  does 
not  remain  upon  the  glands  as  in  the  majority  of  flowers,  but 
runs  down  through  a  narrow  duct  between  the  filaments  and 
collects  in  the  spur-like  prolongation  of  the  corolla  (Fig.  333,  s). 


FIG.  333.  Examples  of  the  Figwort  family:  A,  inflorescence  of  the  toad- 
flax (Linaria).  B,  flower  viewed  from  beneath,  showing  the  under  lip  arching 
up  against  the  upper,  two-lobed  lip — s,  nectar  spur  of  the  corolla.  C,  section 
of  the  flower  viewed  from  the  side,  showing  the  undivided  ovary  with  central 
ovules.  Note  the  stigma  and  anthers  concealed  at  the  lips  of  the  corolla. 
D,  sectional  view  of  flower  of  Rhinanthus  with  the  four  stamens  arranged 
in  pairs.  E,  appearance  of  the  stamens  as  viewed  from  the  mouth  of  the 
corolla.  The  filaments  have  been  pressed  apart,  thus  separating  the  opened 
anthers. 

The  yellow  rattlebox  (Rhinanthus,  Fig.  333,  D,  E)  shows  an- 
other form  of  the  flower,  common  in  this  family.  The  general 
arrangement  of  the  floral  organs  is  the  same  as  in  Linaria,  but 
the  anthers  are  in  pairs  so  that  when  they  open,  owing  to  the 
rigidity  of  the  filaments  or  the  pressure  of  the  upper  lip,  they 


480  THE   POLEMONIALES 

are  kept  tightly  pressed  together  like  a  pair  of  sugar  tongs, 
thus  preventing  the  shedding  of  the  spores.  Owing  to  the  form 
of  the  corolla  and  often  because  of  numerous  hairs  and  bristles, 
the  insect  is  directed  in  such  a  definite  way  to  the  flower  that  he 
comes  in  contact  with  appendages  of  the  anthers,  or  causes  a 
slight  deflection  of  the  rigid  filaments  or  of  the  lip  of  the  corolla, 
any  of  which  movements  are  sufficient  to  separate  the  anthers 
and  bring  down  upon  him  a  shower  of  spores.  These  flowers 
are  largely  pro togy nous  so  that  at  first  only  a  crossing  is  possible. 
In  many  species,  autogamy  results  from  the  downward  bending 
of  the  corolla  and  style,  accompanied  by  a  loosening  of  the  anthers 
so  that  during  the  last  days  of  flowering  the  spores  may  fall 
directly  upon  the  stigma. 

Many  of  these  flowers  are  characterized  by  mottlings  and 
blotches.  It  appears  to  be  a  rather  general  law  that  the  most 
highly  modified  parts  are  variegated  in  this  manner — compare  the 
mints  and  orchids.  Less  specialized  flowers  present  an  associa- 
tion of  various  colors  or  veinings,  a  feature  that  is  illustrated 
in  many  families,  as  violets,  peas,  geraniums,  etc.,  whereas  the 
simplest  types  of  flowers  are  usually  of  a  uniform  color.  The 
figworts  have  been  called  a  suspicious  group,  and  if  not  actually 
poisonous,  none  at  least  serve  as  food.  Several  are  medicinal, 
as  Digitalis,  Veronica,  Gratiola,  etc.  Many  are  parasitic,  as  the 
foxglove,  Gerardia,  eyebright  (Ettphrasia)  and  the  related  and 
reduced  brownish  rapes  (Thalesia  and  Orobanche),  beech  drops 
(Leptamnium)  and  squawroot  (Conopholis) . 

The  curious  family  of  the  bladderworts  is  closely  related  to 
the  figworts.  The  bladderwort  (Utricularia)  is  a  very  common 
aquatic  in  still  waters  and  ponds.  Certain  leaves  become  modi- 
fied into  elaborate  sacs  with  trapdoors  for  enticing  and  capturing 
small  insects  which  die  and  decay  in  these  prisons  and  are  ulti- 
mately absorbed  and  so  contribute  to  the  nourishment  of  the 
plant.  The  butterwort  (Pinguicula)  is  another  member  of  this 
family  found  in  the  northern  countries  and  also  in  our  southern 
states.  It  has  more  the  appearance  of  a  violet  with  a  rosette 
of  leaves  which  are  provided  with  glands  that  secrete  a  viscid 
substance  for  the  capture  of  insects  and  also  digestive  fluids. 


DEVELOPMENT  OF   PLANTS  481 

The  contact  of  the  insect  acts  as  a  stimulus,  causing  the  leaves 
to  roll  up  and  so  bringing  to  bear  upon  him  a  larger  surface  of 
digestive  glands.  One  of  the  species  of  Pinguicula  is  used  in 
northern  countries  to  curdle  milk  in  place  of  "rennet"  which 
contains  the  similar  digestive  fluids  of  calves'  stomachs. 

The  Potato  family  (Solanaceae)  shows  many  of  the  irregu- 
larities of  the  figworts,  but  the  majority  of  its  1,700  species  have 
regular  flowers  as  illustrated  in  Fig.  334.  While  the  family 


FIG.  334.     Flower  of  the  potato  (Solanum  tuberosum),  showing  the  stamens 
inserted  on  the  tube  of  the  corolla  and  encircling  the  style. 

is  largely  tropical,  many  of  the  species  are  cultivated,  as  the 
potato,  eggplant,  tomato,  cayenne  pepper  (Capsicum).  Poison- 
ous, acrid  and  narcotic  properties  are  characteristic  features  of 
the  family.  Belladonna  and  atropine  from  Atropa,  stramonium 
from  Datura  and  nicotine  from  Nicotiana  Tabacum  are  character- 
istic drugs.  Ground  cherry  (Phy satis),  nightshade  (Solanum), 
Petunia,  etc.,  are  cultivated  forms. 

153.  A  Transitional  Order. — It  would  appear  that  all  the 
changes  possible  in  the  hypogynous  type  of  flowers  had  been 
wrought  in  the  members  of  the  Polemoniales  and  that  the  next 
step  in  advance  must  be  to  the  epigynous  flower.  This  is  seen 
to  be  the  case  with  the  madder  order,  Rubiales.  Here  the 
flowers  are  epigynous  and  the  parts  are  generally  in  fives  though 
the  carpels  vary  greatly  in  number.  The  simplicity  in  the  struc- 
ture of  the  flower  and  often  also  the  form  of  the  inflorescence, 
as  in  the  elderberry  (Sambucus),  bedstraw  (Galium)  and  arrow- 
wood  (Viburnum)  is  strikingly  suggestive  of  the  Umbellales 
(Figs.  335,  A,  B\  324).  It  appears  that  each  step  in  advance 
is  attained  in  a  very  simple  way  and  that  only  gradually  are 


482 


THE   RUBIALES 


alterations  made  that  lead  up  to  the  specialized  types.  Look 
back  over  the  preceding  groups  and  orders  and  note  that  many 
of  the  irregular  types  of  flowers  are  preceded  by  a  long  series 
of  regular  forms.  So  in  the  madder  order,  this  new  departure 
of  the  flower  is  attended  with  such  a  simplicity  of  structure  as 
to  render  difficult  the  separation  of  some  of  the  genera  from  the 
open  flowers  of  the  Umbellales  which  have  also  arrived  at  the 


FIG.  335-  Simpler  forms  of  the  Rubiales:  A,  inflorescence  of  the  arrow- 
wood  (Viburnum}.  B,  simple,  epigynous  flower  of  the  elderberry  (Sam- 
bucus) — o,  ovary.  C,  the  twin  flower  (Linnaea). 

same  stage  of  development,  page  465.  This  would  not  necessarily 
imply  a  relationship,  since  it  repeatedly  happens  both  among 
plants  and  animals  that  identically  similar  structures  arise  in 
groups  in  no  way  related.  From  these  simple  flowers,  that  may 
be  no  more  sympetalous  than  certain  genera  of  the  carrot  order, 
we  pass  to  more  pronounced  tubular  forms  in  t^ie  bluets  (Hous- 
tonia),  buttonbush  (Cephalanthus) ,  twin  flower  (Linnaea,  Fig. 
335 1  Oi  snowberry  (Symphoricarpus)  and  finally  to  irregular 
and  even  labiate  types  as  in  the  honeysuckle  (Lonicera,  Fig. 
268),  valerian,  teasel  (Dipsacus)  and  scabious  (Fig.  336).  It 
is  noteworthy  that  in  the  three  latter  genera  the  flowers  become 
massed  in  a  dense  inflorescence,  known  as  a  head,  which  are  sub- 


DEVELOPMENT  OF   PLANTS 


483 


tended  by  modified  leaves,  the  involucre  (Fig.  336)  and  the  pistils 
are  reduced  to  a  single  fertile  carpel  with  two-lobed  style.  These 
variations  will  become  very  prominent  in  the  next  order.  The 
Rubiales\re  an  important  tropical  group  and  furnish  the  coffee 
(Coffea)  and  the  cinchonas  which  yield  such  drugs  as  quinine 
and  calisaya. 


FIG.  336.  Advanced  forms  of  the  Rubiales:  A,  inflorescence  of  Scabiosa, 
at  the  right  showing  the  involucre,  in.  B,  a  single  flower  enlarged,  show- 
ing the  somewhat  irregular  corolla.  At  the  right  the  sectional  view  of  the 
flower  shows  the  calyx,  ca,  terminating  in  bristle-like  teeth;  br,  bract-like 
cup  surrounding  the  calyx.  C,  irregular  flower  of  valerian — n,  nectar  sac; 
ca,  rudimentary  calyx,  which  matures  in  the  fruit  as  a  mass  of  delicate  feath- 
ery organs,  known  as  the  pappus. 

154.  Campanulales,  the  Bellflower  Order. — This  order  marks 
the  culmination  of  the  tendencies  that  we  have  seen  steadily 
progressing  through  the  monocotyledons  and  the  dicotyledons. 
The  variations  have  been  of  so  peculiar  and  successful  a  nature 
that  no  group  of  plants  are  so  widely  distributed  and  in  the 
more  specialized  families  of  so  common  occurrence.  Over 
14,500  species  are  known.  The  parts  of  the  epigynous  flowers 
are  arranged  in  four  whorls  of  usually  five  members  each.  The 


484 


THE   CAMPANULALES 


anthers  are  aggregated  and  usually  cohere,  forming  a  sheath 
about  the  style  which  is  frequently  covered  with  hairs  and  acts 
like  a  piston  rod  in  the  cylinder  of  anthers.  The  flowers  are 
protandrous  with  few  exceptions,  the  anthers  opening  on  their 
inner  sides  and  discharging  the  spores  upon  the  style  which  later 
sweeps  them  out  as  it  elongates.  The  pistils  are  generally  re- 
duced to  a  single  one-ovuled  carpel  that  ripens  as  an  akene. 
Leaving  out  of  consideration  the  gourds  which  include  the 
melons,  pumpkins,  cucumber  and  gourd,  as  a  family  of  uncertain 


FIG.  337.  Lower  forms  of  the  Campanulales:  A,  inflorescence  of  the 
bellflower  (Campanula}.  B,  section  of  a  young  flower.  The  hairy  style 
is  pushing  up  between  the  encircling  anthers  and  sweeping  the  spores  out 
of  them.  Note  the  closed  stigmatic  lobes.  C,  older  flower.  The  anthers 
are  withering  and  the  stigmas  are  curving  back  towards  the  spore-covered 
style.  D,  flower  of  Lobelia.  The  tubular  corolla  is  opened  at  one  side,  per- 
mitting the  style  and  encircling  stamens,  g,  to  protrude.  E,  section  of  flower, 
showing  the  anthers,  a,  united  about  the  style  and  stigma.  F,  relation  of 
stigma  to  anthers.  At  right  the  section  shows  the  anthers  cohering  about  the 
bushy  style,  which  acts  later  as  a  brush  sweeping  out  the  spores.  At  the  left 
the  style  has  grown  beyond  the  anthers  and  the  stigmatic  lobes  are  spreading 
apart. 


DEVELOPMENT  OF  PLANTS  485 

alliance,  we  find  a  very  natural  sequence  leading  to  the  higher 
families.  For  example  the  lower  members  of  the  Bell-flower 
family  have  nearly  or  quite  regular  flowers,  parts  in  five,  anthers 
grouped  about  the  style  but  not  united,  fruit  a  capsule  with  many 
seeds  as  in  the  harebell  (Campanula)  and  Specularia  (Fig.  337, 
A,  C).  In  the  Lobelia  the  corolla  usually  becomes  irregular 
and  slit  down  one  side,  thus  approaching  the  form  assumed  in 
the  highest  groups  of  the  order.  The  anthers  are  united  about 
the  style  which  is  two  lobed  and  provided  with  a  whorl  of  hairs 
to  sweep  the  spores  from  the  cylinder  of  anthers.  The  fruit  is 
a  capsule  of  two  carpels  (Fig.  337,  D-E).  This  brings  us  to 
that  great  group  of  familiar  plants  that  were  formerly  known  as 
the  Compositae,  but  that  are  now  separated  into  the  Chicory 
family  (Cichoriaceae)  of  1,400  species,  the  Ragweed  family 
(Ambrosiaceae)  of  55  species,  and  the  Thistle  family  (Cardu- 
aceae)  of  over  11,000  species.  The  most  conspicuous  feature 
of  these  three  families  is  the  aggregation  of  numerous  small 


FIG.  338.  Habit  of  the  dandelion  (Taraxacum},  an  example  of  the  Cicho- 
riaceae: a,  appearance  of  the  inflorescence  or  head;  b,  appearance  of  the  head 
during  the  ripening  of  the  seed;  in,  involucre;  c,  appearance  of  the  fruit. 


486 


THE   CAMPANULALES 


flowers  in  heads  subtended  by  one  or  more  rows  of  bracts  that 
form  a  calyx-like  involucre  (Fig.  338,  in).  This  type  of  in- 
florescence might  readily  be  mistaken  for  a  single  flower  as  the 
buttercup,  rose,  etc.  This  tendency  to  group  the  flowers  in 
heads  and  compact  clusters  has  been  attained  in  several  orders, 
notably  the  mustards,  peas,  Umbelliferae,  mints,  scrophularias, 
and  especially  in  the  Teasel  family,  page  482.  But  in  no  group 
has  the  aggregation  been  so  successful  and  coupled  with  such 
efficient  types  of  flowers.  Leaving  out  of  consideration  the 
degenerate  ragweeds  the  individual  flowers  of  a  head  are  very 


F!G.  339-  Flowers  and  fruit  of  Taraxacum:  A,  sectional  view  of  inflor- 
escence— in,  involucre.  The  flowers  in  the  center  of  the  head  not  as  yet  in 
bloom.  B,  an  unopened  flower.  The  thread-like  calyx  (pappus)  and  corolla 
arising  from  the  ovary,  o.  C,  corolla  opening.  D,  later  stage,  the  style  has 
elongated,  sweeping  the  spores  from  the  sheath  of  anthers,  an,  and  the  two 
stigmatic  lobes  are  beginning  to  open.  E,  flower  in  full  bloom,  the  stigma 
lobes  recurving.  F,  mature  fruit.  The  pappus  is  lifted  up  on  a  long,  slender 
outgrowth  of  the  ovary,  o. 

uniform  in  structure.  They  are  epigynous,  parts  usually  in  fives, 
calyx  wanting  or  more  often  appearing  as  tufts  of  hairs,  plumose 
or  barbed  bristles,  and  known  as  the  pappus  (Figs.  339,  341). 
The  corolla  is  tubular  or  partially  split  open,  forming  a  strap- 


DEVELOPMENT  OF  PLANTS  487 

shaped  blade  known  as  the  ligulate  corolla  (Fig.  339,  E).  The 
anthers  are  united  about  the  style  which  is  often  hairy  and  pushes 
out  the  microspores  like  a  piston  (Fig.  341,  B-D).  The  style 
is  two-lobed,  the  stigma  usually  appearing  as  a  line  on  the  inner 
surface  of  the  lobes  and  the  single,  one-ovuled  carpel  matures 
as  an  akene.  Scale-like  bracts,  the  chaff,  are  often  associated 
with  the  flowers  (Fig.  341,  B).  The  nectar  formed. from  the 
glands  at  the  base  of  the  style  is  concealed  at  the  bottom  of  the 
corolla  tube. 

(a)  The  Chicory  Family. — This  family  is  distinguished  by  all 
the  flowers  of  the  head  being  ligulate  (Fig.  339)  and  mostly  yel- 
low in  color  and  by  the  possession  of  milky,  bitter  or  acrid  juices. 
Here  belong  the  goat's  beard  (Adopogon),  hawkbit  (Leontodori) , 
sow  thistle  (Sonchus),  hawkweed  (Hieracium),  rattlesnake  root 
(Nabalus),  as  well  as  several  introduced  and  native  plants  that 
are  cultivated,  as  the  dandelion  (Taraxacum),  species  of  chicory 
(Cichorium),  salsify  (Tragopogon) ,  lettuce  (Lactuca).  The  char- 
acter of  the  head,  flower  and  fruit  of  this  family  is  well  shown 
in  the  dandelion  (Fig.  339).  All  the  flowers  are  ligulate,  the 
five  lobes  frequently  to  be  seen  on  the  margin  of  the  corolla 
indicating  the  number  of  petals,  and  the  calyx  assumes  the  form 
of  minute  hairs  (Fig.  339,  B-E).  The  five  filaments  adhere  to 
the  tube  of  the  corolla  and  the  anthers  form  a  cylinder  about 
the  style  and  discharge  their  spores  before  the  flower  opens.  The 
style  begins  to  elongate  as  soon  as  the  flower  opens  and  pushes 
out  the  spores  which  may  be  seen  as  little  piles  of  dust  at  the 
top  of  the  anthers  in  freshly  opened  flowers.  The  filaments  of 
this  genus  are  sensitive  to  touch  and  curve  outward,  pulling  down 
the  sheath  of  anthers,  thus  assisting  in  sweeping  out  the  spores 
and  exposing  the  style  when  an  insect  irritates  them  in  his  quest 
for  nectar.  This  device  appears  in  other  genera  and  is  sometimes 
the  only  means  of  exposing  the  microspores  and  styles.  Autog- 
amy is  at  first  impossible  owing  to  the  fact  that  the  two  lobes  of 
the  style  are  closely  pressed  together  effectually  excluding  the 
microspores  from  its  inner,  stigmatic,  surface.  Later  the  style 
grows  considerably  above  the  anthers  and  the  lobes  separate,  ex- 
posing the  stigmas  for  crossing  (Fig.  339,  E).  The  association 


488  THE   CAMPANULALES 

of  this  type  of  flower  is  more  effective  than  any  as  yet  noticed, 
and  it  would  appear  impossible  for  any  of  the  numerous  insects 
which  frequent  these  flowers  to  miss  crossing  a  score  of  them  at 
a  single  visit.  Crossing  between  the  flowers  of  a  head  is  effected 
in  some  genera  by  the  movements  of  the  flowers.  The  inner 
leaves  of  the  involucre  fold  over  the  flowers  at  night  and  in  rainy 
weather,  protecting  them  like  a  perianth.  This  results  in  crowd  - 


FIG.  340.     Inflorescence  of  the  bur-marigold  (Bidens),  a  common  ray  form 
of  flower  of  the  Thistle  family:  in,  involucre. 

ing  of  the  flowers  together,  and  the  stigmas  are  often  brought  in 
contact  with  the  spores  that  have  been  dusted  upon  the  various 
parts  of  adjoining  flowers.  The  outward  curving  of  the  stylar 
lobes  may  also  bring  the  stigmas  in  contact  with  the  spore-covered 
parts  of  adjoining  flowers  with  a  like  result.  Autogamy  is 
brought  about  in  the  dandelion,  as  in  nearly  all  other  members 
of  the  order,  by  the  curvature  of  the  lobes  of  the  style  which 
continue  to  bend  back  until  the  stigmatic  surface  is  brought  in 
contact  with  the  spore-covered  style  (Fig.  341,  D).  The  bloom- 
ing of  the  flowers  progresses  from  the  margin  to  the  center  of 
the  head  so  that  during  several  days  new  sets  of  flowers  are 
exposed  for  crossing.  When  the  period  of  bloom  is  passed  the 
involucre  closes  over  the  head  and  remains  in  this  condition  until 
the  fruit  is  mature  (Fig.  338,  &).  During  this  period,  the  stalk 


DEVELOPMENT   OF   PLANTS  489 

bearing  the  head  elongates  considerably,  so  as  to  lift  the  fruit 
above  the  surrounding  vegetation — the  extent  of  its  elongation 
depends  upon  the  height  of  this  vegetation.  The  corolla  withers 
away  and  the  hair-like  calyx  grows  out  into  a  white,  delicate 
pappus,  which  is  lifted  upon  a  beak-like  outgrowth  of  the  ovary 
(Fig-  339>  ^)-  The  fruit  is  now  mature  and  in  a  position  for  dis- 
tribution. The  involucre  opens,  the  hairs  of  the  pappus  expand, 
loosening  the  akenes  so  that  the  least  touch  or  breath  of  air  floats 
off  the  fruit  as  the  most  perfect  type  of  parachute  (Fig.  338,  c). 

(b)  The  Thistle  Family,  Carduaceae. — This  is  the  largest 
family  of  the  angiosperms.  The  flowers  have  the  same  arrange- 
ment and  structure  as  in  the  Cichoriaceae,  save  that  the  corollas 
are  either  all  tubular,  as  in  the  thistle  and  ironweed  (Fig.  342), 
or  the  marginal  flowers  of  the  head  may  be  ligulate,  thus  in- 
creasing the  conspicuousness  of  the  inflorescence,  as  in  the  asters, 
daisy,  etc.  (Figs.  340,  341).  The  marginal  ligulate  flowers  are 
termed  ray  flowers  to  distinguish  them  from  the  inner  tubular  or 
disc  flowers.  The  ray  flowers  may  be  sterile,  though  more  fre- 
quently imperfect  and  provided  with  pistils.  The  calyx  may  be 
wanting,  but  more  often  assumes  the  form  of  silky  or  plumose 
pappus  or  of  membranous  scales  for  wind  transportation  or  of 
barbed  bristles  of  various  kinds  for  distribution  by  animals.  In 
the  case  of  the  burdock  (Arctium)  the  involucre  is  covered  with 
hooks.  The  Carduaceae  includes  a  great  array  of  plants,  many 
of  which  are  among  our  troublesome  weeds :  Ironweed  ( Ver- 
nonia),  thoroughwort  (Eupatorium) ,  blazing  star  (Lacinaria), 
golden-rod  (Solidago),  aster,  fleabane  (Erigeron),  cat's-paw 
(Antennaria) ,  everlasting  (Gnaphalium) ,  rosinweed  and  com- 
pass plant  (Silphium),  Spanish  needles  (Bidens),  daisy  (Chrys- 
anthemum), groundsel  (Senecio),  thistle  (Carduus).  Cultivated 
for  their  showy  flowers:  Tickseed  (Coreopsis),  cone  flower  (Rud- 
beckia),  sunflower  (Helianthus) ,  cornflower  (Centaur  ea),  ciner- 
arias, etc.  Medicinal:  Wormwood  (Artemisia) ,  one  species  of 
which  yields  absinth,  milfoil  (Achillea),  colt's  foot  (Tussilago) , 
tansy  (Tanacetum),  chamomile  (Anthemis),  arnica,  burdock 
(Arctium). 

The  Ragweed  family,  Ambrosiaceae,  is  a  small  group,  princi- 
32 


490 


THE   CAMPANULALES 


pally  North  American  in  its  distribution,  that  has  become  sepa- 
rated from  the  Carduaceae  through  degeneration.  The  heads 
contain  a  few  greatly  reduced  wind-pollinated  flowers  that  are 
always  imperfect  and  generally  lacking  in  calyx  or  corolla,  or 


FIG.  341.  Flowers  and  fruit  of  Bidens:  A,  sectional  view  of  the  inflor- 
escence— r,  ray  flowers;  d,  disc  flowers;  in,  bracts  of  the  involucre.  B,  disc 
flower  before  opening,  the  shaded  region  showing  the  position  of  the  anthers 
in  the  corolla — c,  calyx  in  the  form  of  downwardly  barbed  bristles;  b,  bract 
or  chaff  associated  with  the  disc  flowers.  C,  early  stage  in  the  opening  of 
the  flower.  The  stamens  have  been  lifted  beyond  the  mouth  of  the  opened 
corolla  by  the  growth  of  their  filaments  and  the  style  is  elongating,  pushing 
out  the  spores,  which  appear  in  little  piles  at  the  top  of  the  anthers.  D,  last 
stage  in  the  bloom  of  flowers.  The  style  has  grown  beyond  the  anthers  and 
the  stigmatic  lobes  have  reflexed,  touching  the  spore-covered  style.  E,  a 
sterile  ray  flower,  much  less  enlarged.  F,  the  fruit,  showing  the  barbed  pappus 
for  dissemination. 


DEVELOPMENT  OF   PLANTS 


491 


both,  and  characterized  by  free  anthers.  The  more  familiar  are: 
The  marsh  elder  (Iva),  ragweed  (Ambrosia),  and  clot-bur  and 
cockle-bur  ( Xantkium) . 

These  three  alliances  are  apparently  the  most  recently  evolved 
of  the  angiosperms,  and  owing  to  their  numerous  variations  that 
have  been  so  successful  in  meeting  the  present  conditions  upon 


FIG.  342.  A  common  tubular  flower  of  the  Thistle  family:  A,  inflores- 
cence of  the  ironweed  (Vernonia).  B,  sectional  view  of  the  inflorescence, 
only  the  outer  flowers  in  bloom.  C,  enlarged  view  of  one  of  the  flowers — ct 
calyx  or  pappus.  D,  fruit. 

the  earth,  they  have  become  the  dominant  plants  the  world  over. 
Their  success  can  be  traced  to  a  variety  of  causes,  as  the  occur- 
rence in  many  genera  of  perennial  underground  stems,  which 
make  them  to  a  degree  independent  of  climatic  conditions  and  the 
depredations  of  grazing  animals.  A  great  many  are  avoided 
because  of  their  protective  armor  of  prickles,  bitter,  acrid  juices 
or  oils  and  resins.  Especially  to  be  noted  is  the  seed-like  fruit 
with  its  numerous  devices  for  distribution.  All  these  features, 
among  others,  explain  their  extensive  distribution  in  open  coun- 
tries. Another  advantage  appears  in  the  structure  of  the  flower 
and  the  inflorescence.  The  highest  type  of  a  flower,  as  in  the  case 
of  a  machine,  is  the  one  that  is  most  efficient.  The  flower  is 
not  so  elaborate  as  the  orchid  or  as  those  of  some  other  groups, 
but  it  is  superior  because  it  accomplishes  its  work  with  greater 
certainty  and  more  economically.  Note  the  significance  of 
these  facts — the  ovary  of  the  orchid  consists  of  three  carpels 


492  THE   CAMPANULALES 

which  may  produce  thousands  of  seeds,  whereas  the  simple 
ovary  of  one  of  the  Compositae  contains  but  a  single  seed. 
Reduction  has  been  made  in  every  part  without  loss  of  con- 
spicuousness  and  variations  have  also  appeared  that  practically 
eliminate  the  uncertainties  of  crossing  and  autogamy,  and  con- 
sequently the  formation  and  distribution  of  the  seed  are  ensured. 
In  a  word  every  evolutionary  tendency  that  you  have  noticed 
in  the  development  of  the  flower  finds  its  expression  in  tnis 
group  of  plants. 


INDEX 


Abies,  376 

Absorption,  12,  20,  51,  54 

Acer,  457,  459 

Accompanying  cells,  79,  80 

Achlya,  219 

Actinomorphic,  387 

Adhesion,  385,  471 

Adhesive  discs,  108,  109 

Adiantum,  329 

Adlumia,  443 

Aecial  stage,  252 

Aeciospores,  252 

Agaricaceae,  259,  260 

Agaricales,  259 

Agrimony,  450 

Air  chambers,  273,  282,  283,  336,  402 

Akene,  398,  438,  440 

Albugo,  222,  223 

Alder,  432 

Algae,  173 

blue-green,  164,  233 

brown,  201 

colonial,  178 

conjugating  green,  185 

filamentous  green,  190 

green,  174 

red,  210 

tubular  green,  195 

unicellular  green,  174 
Alga-like  fungi,  218 
Allen,  215 
Allelomorph,  141 
Alnus,  432 

Alternation  of  crops,  67 
Alternation  of  generation,  136,   137, 
194,  201,  211-216,  256,  271,  279, 
281,  312,  319.352 
Amanita,  261 
Amaryllis  family,  419 
Ambrosiaceae,  485,  489 
Ament,  429,  430 
Ammonia,  66,  159,  160 


Angiospermae,  378 
Ankistrodesmus,  182 
Annual  rings,  of  scars,  75,  73 

of  xylem,  91,  92 
Annuals,  69 
Annulus,  261,  262,  300,  301,  306,  307 

316,  326,  327,  355 
Anther,  116,  117,  379 

versatile,  408 
Antheridial  cell,  121,  122,  358,  369, 

370,  392 
Antheridium,   antheridia,    196,    208, 

274,303 

Antherozoids,  196,  275 
Anthoceros,  292,  294 
Anthocerotales,  292 
Antipodal  cells,  119,  120,  390 
Antitoxin,  164 
Apophysis,  305,  306 
Apple,  450,  451 

rust,  256,  257 
Aquatic  plants,  39,  41,  64 
Arales,  410 
Aralia,  30,  465 
Araliaceae,  465 
Archangiopteris,  316 
Archegonium,  275,  276,  303,  350 
Arcyria,  150,  151 
Aroid  order,  410 
Arrow- wood,  481,  482 
Arum,  411,  413 
Ascocarp,  229 
Ascomycetes,  228 
Asco-lichens,  232 
Ascopores,  229 
Ascus,  asci,  229,  230 
Asexual   generation,    136,    137,    194, 

204,  214,  256,  271,  279,  281,  319, 

320 

Asexual  spore,  123,  172 
Aspergillales,  240 
Aspergillus,  241,  242 


493 


494 


INDEX 


Asplenium,  324 
Autoecious,  251 
Autogamy,  383,  439,  446,  454,  470, 

480,  488 
Azalea,  471,  472 

Bacillus,  155 
Bacteria,  154,  155 

economic  importance  of,  157 

exclusion  of,  156 

of  decay,  157,  158 

of  disease,  157,  162 

iron,  161 

nitrifying,  65,  67,  160 

nitrogen  fixing,  161 

sulphur,  161 

synthetic,  159 
Banana,  420 
Banyan  tree,  70 
Barberry  rust,  251 
Bark,  86,  87 
Basidial  stage,  256 
Basidiomycetes,  251 
Basidiospore,  255,  256 
Basidium,  basidia,  251,  263,  255,  258, 

262,  263 

Basswood  stem,  100 
Bean  seed,  130,  131 
Beard,  44 
Beech,  431,  434 

ordei,  431 
Begonia,  2 
Bellflower,  484 

order,  483 
Berry,  398 
Bidens,  488,  490 
Biennials,  69 
Bilabiate  flowers,  478 
Birch  leaf,  7,  427 

family,  147,  431,  432 
Black  knot,  239,  240 
Bladderwort,  480 
Bladderwrack,  206 
Blade,  7,  8,  21,  27,  426 
Blakeslee,  227 
Bleeding,  of  plants,  61 
Bloodroot,  442,  443 
Blueberry,  471,  472 


Blue  green  algae,  165 

Body  cell,  358,  369,  370 

Bogs,  296,  473 

Boletaceae,  265 

Boletus,  265 

Borages,  476 

Boraginaceae,  475,  476 

Botrychium,  314,  315,  316,  317 

Bower,  326 

Brassica,  445 

Bread  making,  20,  250 

Bryales,  301 

Bryophyta,  269 

Buckthorn  order,  461 

Buckwheat  order,  437 

Buds,  71-75,  73,  362 

adventitious,  75 

axillary,  74 
Bulbs,  113 
Buller,  264 

Bunch  flower  family,  418 
Bur-marigold,  488,  490 
Burroughs,  415 
Buttercup,  438 

order,  438 
Butternut,  433,  435 
Butterwort,  480 

Cabbage,  447 
Cactus,  44 

Calcium. cyanamide,  66 
Callithamnion,  213 
Callus,  75,  101 
Calyptra,  294,  299,  305 
Calyx,  115,  116,  384 
Cambium,  79,  80,  82,  89,  314 

cylinder,  89,  90,  91,  314,  426 
Campanula,  484 
Campanulales,  483 
Canal  cells,  276,  303,  317,  318,  345 
Canna,  421 

Capillitium,  150,  151,  153 
Capsule,  277,  305,  306,  307,  326,  414. 

417 

Carbohydrates,  II,  12 
Carbon,  15 

dioxide,  12,  15,  21,  49,  159 
Carbonic  acid,  13 


INDEX 


495 


Carduaceae,  485,  489 
Carex,  409 
Carpel,  115,  116,  399 
Carpinus,  432 
Carrot,  order,  465 

family,  465 

wild,  466 

Caruncle,  132,  133 
Cassia,  35,  451,452 
Castor  bean,  76,  77,  90,  91,  133 
Catkin,  429,  430 
Cat-tail,  381,  401,  403 

order,  401 

Cedar  apple,  256,  257 
Celery,  blanching  of,  14 
Cell,  crystals,  2 

division  of,  56,  57,  58 

intercalary,  253,  254 

nature  of,  I 

sap,  I,  3,  33 

wall,  i,  57,  58 
Cellulose,  n 
Ceramium,  211 
Cercis,  451,  452 
Chaetophorales,  190 
Chaff,  487,  490 
Chamaecyparis,  376 
Chamaenerion,  463,  464 
Characters,  nature  of,  144 
Chemical  reaction,  3 
Chemo-synthesis,  162 
Chenopodiales,  436,  437 
Chenopodium,  436 
Cherry,  450,  451 
Chervil,  468 

Chicory  family,  485,  487 
Chlamydomonas,  178,  179 
Chlorenchyma,  n,  12,  76 
Chlorophyceae,  174 
Chlorophyll,  13-15,  167 
Chloroplasts,  2,  9,  10,  12,  13,  17 
Chondrus,  214 
Choripetalae,  429 
Chromatin,  56,  57 
Chromoplasts,  2,  32 
Chromosome,  56,  57 

reduction  of,  136,  137,  214-216, 
280 


Chromosome,  homologous,  141 

Cichoriaceae,  485,  487 

Cilium,  151,  152 

Cladonia,  233 

Class,  147 

Classification  of  plants,  146 

Clavaria,  264 

Clavariaceae,  264 

Claviceps,  237,  238 

Climbing  plants,  106 

Closterium,  188 

Club  moss,  341 

Club  root,  153 

Cluster-cup,  252 

Cohesion,  385 

Coenocytes,  195 

Coleochaetes,  199,  200 

Collateral  bundles,  314,  354 

Collecting  cells,  9,  21 

Collenchyma,  78,  83,  84 

Colors  of  flowers,  417,  423,  441,  446. 

480 

Columbine,  440 
Columella,  225,  293,  294 
Comfrey,  385,  475,  476 
Commensalism,  68,  234 
Compass  plant,  33,  34 
Compositae,  485 
Concentric  bundles,  323,  324 
Conducting  system,  82,  94,  312 
Cone,  337,  342,  366 
Conidium,  228 
Conn,  156 
Consumption,  163 
Contact,  reaction  to,  63,  106,  107 
Cordyceps,  238,  239 
Cork,  42,  76,  86,  87,  314-315 

cambium,  86,  87 
Corm,  112,  113 
Corn,  root  of,  57 

seedling  of,  134,  135 
Corolla,  115,  116,  384 
Cortex,  59,  60,  61,  76,  314-315 
Corylus,  432 
Coscinodiscus,  169 
Cosmarium,  189 

Cotyledon,  124,  125,  126,  131,    132, 
133,  3i8,  332,  360,  398,  428 


496 


INDEX 


Creeping  stems,  109 

Cribraria,  150 

Crossing,  115,  138,  382,  404,  412,  413, 

445,  446,  463,  467,  476,  487 
Crowfoot  order,  438 
Crucibulum,  267 
Cruciferae,  444,  445 
Currant,  387,  449 
Cuticle,  10,  25,  36,  269 
Cutin,  10,  36 
Cyanophyceae,  165,  166 
Cycadales,  353 
Cycads,  353 
Cycas,  353,  355 
Cyclic  flowers,  384,  385 
Cymbella,  170 
Cyperaceae,  410 
Cypripedium,  424 
Cystocarp,  212,  213 
Cytoplasm,  2 

Daldinia,  239,  241 

Dandelion,  485,  486 

Darwin,  64,  132 

Dasya,  211 

Daucus,  466 

Decay,  158 

Deciduous,  42 

Decompositions,  in  living  substance, 

22-23 

Decussate  leaf  arrangement,  27,  28 
Delesseria,  211 
Dennstaedtia,  328 
Desert  plants,  44 
Desmidium,  188 
Desmids,  188,  189 
De  Vries,  144,  463 
Diadelphous,  452 
Diastase,  20,  250 
Diatomaceae,  167 
Diatoms,  167,  168 
Dichotomy,  207,  271,  272 
Dicotyledons,  96,  125,  426 
Dioecious,  284,  339 
Diphtheria,  164 
Disease,  162 
Disc  flower,  489,  490 
Divisions,  148 


Dodder,  64 
Dodge,  229 
Dominant  factor,  143 
Drupe,  398 
Dryopteris,  324 
Duckweed,  410 
Ducts,  79 

Earth  star,  267 

Ectocarpus,  202,  203 

Egg  cell,  119,  120,  197,  275 

Elaters,  286,  287,  338,  339 

Elderberry,  481,  482 

Elfvingia,  265 

Elm,  436 

Embryo,  124,  125,  359,  360,  396 

cell,  124,  125,  395,  396 

sac,  124,  394,  395 
Empusa,  227 

Endodermis,  59,  61,  77,  316,  325 
Endosperm,  124,  125,  133,  373,  391, 
394.  398 

nucleus,  119,  120,  123,  389,  390, 

394 

Energy  of  plant,  13,  15,  23,  162 

Enzyme,  17,  20,  128,  157 

Epicotyl,  131,  132,  398,  428 

Epidermis,  8,  9,  26,  36,  272 

Epigynous,  386,  387 

Epiphragm,  305,  306 

Epiphyte,  64,  422 

Equisetales,  334 

Equisetum,  335,  339 

Ergot,  237,  238 

Ericales,  470 

Erysiphe,  244.  254,  247 

Erythronium,  383,  414,  415 

Etiolation,  14 

Eucalyptus,  33 

Eumycetes,  216 

Euphyceae,  173 

Evaporation,  25 

Evening  primrose,  29,  463,  464 

Evolution  of  Algae,  174,  190 
of  Angiospermae,  378 
of  Bryophyta,  269,  290,  295,  309 
of  Eumycetes  (true  fungi),  218, 

220 


INDEX 


497 


Evolution  of  flower,  381,  388,  426 
of  Gymnospermae,  353 
of  Pteridophyta,  312,  319 
of  sex,  178,  180,  194,  198,  204, 

340 

of  sexuality,  171,  177,  181,  185, 

204 

Excretion,  20,  22,  53 
Exoascales,  248 
Exoascus,  248 
Exobasidium,  472 
Eye  spot,  175,  176 

Factor,  hereditary,  138 

distribution  of,  139 

dominant,  143 

nature  of,  144 

recessive,  143 
Fagales,  431,  432 
Fagus,  434 
Fairy  rings,  259,  260 
Family,  147 
Farmer,  209 
Fecundation,  122,  123 
Felix,  328 

Fermentation,  157,  158,  249 
Ferments,  see  enzymes. 
Ferns,  311 

adder  tongue,  314,  315 

Christmas,  311,  322 

common,  321 

flowering,  328 

grape,  315 

shield,  324,  325 
Fertilization,  122,  123,  209,  276,  393 

significance  of,  137,  144 
Fertilizers,  of  soil.  66 
Fibrillae,  of  spindle,  57,  58 
Ficus,  435 
Figwort,  478 
Filament,  116,  165,  379 
Filicales,  321 
Flower,  338,  378,  381 

coloration  of,  417,  423,  441,  480 

cyclic,  384,  385 

development  of,  381 

perfect  and  imperfect,  382 

spiral,  383 


Flower,  structure  of,  115,  116 
Foliar  gaps,  314,  325 
Follicle,  398 
Food  of  plants,  n,  48 

distribution  of,  1 6 

storage  of,  17,  18 
Foods,  formation  of,  12 
Foot,  286,  287 
Formaldehyde,  13 
Formative  regions,  58,  60,  98,  99 
Formic  acid,  13,  40 
Forsythia,  28,  29 
Fragaria,  450 
Fruit,  127,  374,  397~399 
Fucus,  206,  207,  208,  209 
Fumitory,  443 
Funaria,  294,  300,  303,  307 
Fungus,  216 

alga-like,  218 

basidia-forming,  251 

bird's  nest,  267 

black,  239,  240 

blue-green,  240    • 

brown,  240 

coral,  264 

cup,  229 

fly,  227 

gill,  263,  265 

pore,  264,  265 

prickly,  264 

sac,  228 
Funiculus,  118,  389,  395 

Gametangium,    192,    196,    202,   203, 

208 
Gamete,  118,  172,  178,  179,  187,  192, 

198,  200,  211,  275 
female,  118,  120,  178,  389 
male,    118,    121,    122,    178,   318, 

332,  342,  358,  392 

Gametophyte  generation,    136,    194, 

215,  274,  276,  281,  310. 

312,    316-317,    320,    330, 

339r  345,  349,  388 

reduction  of,  313,  349-351 

359,  393 

female,  119,  120,  332,  339,  349, 
357,  367,  368,  388,  389 


498 


INDEX 


Gametophyte,  male,   122,   332,   339, 

349.  357.  369,  392 
Gametospore,  118,  122,  172,  201 

germination  of,  123,  124,  187, 
188,  193,  197,  198,  200,  213, 
214,  215,  276,  277,  280,  286, 
293,  304,  308,  309,  318,  332, 
333,  340,  345.  351.  359.  37*, 
395 

Gasteromycetes,  266 
Gemmae,  270,  331 

Generations,  alternation  of,  136,  137, 
194,  201,  211-216,  256,  271,  279, 
281,  319,  352 
Gentian,  474 

order,  473 
Gentianales,  473 
Genus,  146 
Geoglossum,  232 
Geraniales,  460 
Geranium,  384,  461 

order,  460 
Germ  tube,  278 
Gills,  260,  261,  263 
Ginkgo,  362 
Ginseng  family,  465 
Gleditsia,  451,  452 
Gleocapsa,  166 
Goebel,  309 
Goosefoot  order,  437 
Grafting,  101,  102 
Grain,  134,  135,  397,  398,  409 
Graminaceae,  410 
Graminales,  405 
Grape,  461 

blight,  222 

Grass,  406,  407,  409 

family,  410 

order,  405 

Gravity,  effect  on  plant,  62,  104,  106, 

132,  407 

Ground  nut,  455 
Growth,  apical,  56,  98,  200,  270,  304, 

313 

conditions  of,  72,  74,  129 
definite,  72 
nature  of,  3 
of  stem,  89,  103,  106 


Growth  of  root,  56 

relation  of  chemical  changes  to,  4 

rhythm  of,  93 

Growing  point,  56,  98,  270,  313,  346 
Guard  cells,  9,  10,  26 
Gulf  weed,  206 
Gymnospermae,  353 
Gymnosporangium,  256,  257 

Hairs,  plant,  10,  38,  39,  40 

root,  47,  46,  52 
Haustoria,  220,  221 
Hazel,  432 
Hazen,  177 
Head,  482,  485 
Heath  order,  470 
Heartwood,  95 
Helleborus,  440 
Helvellales,  231,  232 
Hepaticae,  271 
Hepatics,  leafy,  288 
Hepatics,  thallose,  271,  272 
Hereditary  substance,  138 
Heterocyst,  166,  167 
Heteroecious,  251 
Heterogamous,  178 
Heterospory,  347,  352' 
Heterostyly,  462,  463 
Hickory,  fruit  of,  435 

branch  of,  73 

leaves  of,  34 
Hilum,  129,  130 
Honey  locust,  451,  452 
Honeysuckle,  387 
Hop,  twining,  106 
Hormogonium,  166 
Horizontal  branches,  28,  29 
Hornbeam,  432 
Horsechestnut,  31,  34 
Horsetail,  334 
Hybrid,  138 
Hydnaceae,  264 
Hydnum,  264 
Hydrodictyon,  184 
Hydrolysis,  20 
Hydrophytes,  39 
Hymenium,  230,  231,  262,  263 
Hypha,  68,  217 


INDEX 


499 


Hypocotyl,  125,  126,  398,  428 
Hypocreales,  238 
Hypogynous,  386 
Hypoxylon,  239,  240 
Hysteriographium,  241 

Immunity,  164,  245 
Indian-pipe,  69,  472 
Individuality  of  the  plant,  7 
Indusium,  324,  325,  328,  329 
Inflorescence,  381,  382 
Insectivorous  plants,  296,  480 
Insects,  crossing  by,   115,  388,  412, 
413,  417,  419,  421,  424,  439, 

445.  453.  471,  476,  479 

sight  and  sense  of  smell,  417 

Integument,  117,  118,  356,  361,  398, 

425,  428 
Intercellular  spaces,  9,  n,  21,  25,  32, 

273 

Involucre,  285,  287,  466,  483 
Ipomoea,  475 
Iris,  419 

family,  419 
Iron  weed,  491 
Iron  bacteria,  161 
Irritability,  33,  61,  104 
Isogamous,  178 
Isthmus,  189,  190 

Jacket-cells,  356,  357 
Jack-in-the-pulpit,  411 
Jeffrey,  317 
Judas  tree,  451 
Juglandales,  433 
Juglans,  435 
Juncus,  416 
Jungermaniales,  288 
Juniperus,  376,  377 
rust,  256,  257 

Keel,  451-453 
Kelps,  205 
Kerner,  70 
Key  fruit,  398,  459 
Klebs,  185 
Knot,  100 

black,  239,  240 


Labellum,  421,  422,  423,  424,  425 

Labiatae,  476 

Lactuca,  33,  34 

Laminaria,  205 

Larix,  375 

Larkspur,  441 

Lateral  roots,  59,  62,  63 

Latex,  435,  443 

Lathyrus,  453 

Leaf,  arrangement  of,  27-36 

color,  32 

compound,  30,  31 

fall  of,  42,  43 

forms  of,  7,  30,  31 

growth  of,  33 

lobing  of,  30,  31 

movement,  33-36 

reduction  of,  43,  44 

size  of,  24,  25 

structure  of,  7-11,  21,  36-42 

work  of,  1 1-26 
Leaf -scar,  43 
Legume,  65,  398 
Lenticel,  71,  76,  87,  88 
Leotia,  232 
Lepidium,  124,  389,    395,    396,    398 

428 

Lespedeza,  35 
Lessonia,  205 
Leucoplasts,  2,  19 
Lichens,  232 

Asco-,  232,  233 
Licmophora,  168 
Light,  4,  13-14,  17,  31*  36,  41,  63 

104,  106,  112,  176 
Ligulate  flowers,  487 
Lilac  leaf,  9 
Liliales,  414 
Lily,  fawn,  383,  414 

of  the  Valley  family,  418 

family,  418 
Lily  order,  414 
Linaria,  478,  479 
Linin,  56,  57 
Linkage  of  factors,  143 
Linnaea,  482 
Live-forever,  448-489 
Liverworts,  271 


500 


INDEX 


Liverworts,  horned,  292 
Living  substance,  1-6 
Livingston,  183 
Lobelia,  484,  485 
Lockjaw,  bacillus,  155,  163 
Lodicules,  407,  408 
Lomentum,  455 
Loosestrife,  473,  474 
purple,  462,  463 
Lycogala,  150 
Lycoperdales,  266,  267 
Lycoperdon,  266 
Lycopodiaceae,  343 
Lycopodiales,  341 
Lycopodium,  341,  342,  343,  345 
Ly thrum,  462,  463 

Macrocystis,  205 

Madder  order,  481 

Malus,  451 

Maple,  457,  458,  459 

Marattiales,  516 

Marchantia,  274,  282,  284,  285,  286, 

287 

]\larchantiales,  271 
Massee,  245 
Massula,  425 

Mechanical  structure,  78,  83 
Medullary  ray,  94 
Megasporangia,  348,  356,  380,  388, 

389,  391 
Megaspore,  117,  118,  339,  347,  348, 

367,  368,  380,  388,  390 
germination  of,  118,  119 
Megasporophyll,  117,  348,  355,  367, 

379,  38o,  389,  397 
Meibomia,  455 
Melandryum,  383,  437 
Melosira,  168 
Mendel,  142 
Mendel's  Law,  142 
Mesophyll,  9,  10 
Mesophytes,  39 
Metchnikoff,  158 
Micropyle,  117,  118,  355,  356,  368, 

380 

Microsphaera,  247 
Microsporangia,  348,  365,  379 


Microspore,  116,  339,  347,  348 

germination   of,    120,    122,    349 

350,  357,  358,  369,  392,  454 
Microsporophyl,  117,  348,  355,  365 

379 

Micrasterias,  188 
Mildews,  downy,  220,  223 

powdery,  244,  245 
Mimosa,  35,  36 
Minerals  in  soils,  48 
Mint  family,  476 
Mistletoe,  64 
Mnium,  302 
Moccasin  flower,  422 
Monkshood  flower,  441 
Monocotyledons,  96,  125,  400 
Monoecious,  284 
Monotropa,  472 
Morchella,  231,  232 
Morel,  231,  232 
Morning-glory,  475 
Mosaics,  32 
Moss,  295 

bog  or  peat,  296 

chloroplasts,  17 

scale,  290 

true,  301 
Mould,  black,  224,  225,  226 

brown,  240 

blue-green,  217,  240 

water,  218 
Movements,  gases,.  12,  21,  45 

leaf,  33-36 

root,  61 

stem,  104 

water,  24,  50,  54 
Mucorales,  224 
Mushrooms,  217,  259,  260 
Musci,  295 
Mustard  family,  444 
Mutation,  144 
Mycelium,  217 

Mycorrhiza,  68,  243,  296,  317,  422 
Myrtales,  462 
Myrtle  order,  462 
Myxomycetes,  149,  150,  151 
Myxobacteriales,  153 


INDEX 


501 


Narcissus,  419 

Nectar  glands,  381,   388,  404,  430. 

431,437,439,440,457,458 
Nemalion,  213 
Nereocystis,  205 
Netted  veined  leaf,  426,  427 
Nettle,  40,  434 
Nidulariales,  267 
Nitrate,  66,  160 
Nitrite,  160 
Nitrogen,  16,  65,  160 
Nostoc,  166,  292 
Nucleoli,  56,  57 
Nucleus,  i,  2,  56 
Nut,  432,  433 
Nutations,  103,  104,  106 

Oak,  31,383,  427,  434 

Odors,  412,  417,  433 

Oedogonium,  197,  198 

Oenothera,  29,  463,  464 

Olive,  154 

Onoclea,  330 

Oogonium,  197,  208 

Oosphere,  197,  275 

Operculum,  299,  300,  305,  306 

Ophioglossales,  313 

Ophioglossum,  313,  315 

Orange,  460 

Orchid  order,  64,  421 

Orchidales,  421,  423 

Orchis,  425 

Order,  147 

Oscillatoria,  166,  167 

Osmosis,  53 

Osmunda,  326,  328 

Ovary,  116,  117,  380 

Ovule,  116,  117,  118,  380,  388,  389, 

399 
Oxygen,  3,  13,  15,  21,  22,  75,  121 

Palisade  mesophyll,  9,  10,  21,  40,  41 

Pallavicinia,  288 

Palms,  405 

Palmelloid  state,  192 

Pandanales,  401 

Pandorina,  179 

Papaverales,  442 


Papaveraceae,  442 

Papillae  of  stigma,  389,  392 

Papilionaceae,  452 

Papilionaceous  flower,  453 

Pappus,  483,  486 

Paraphyses,  208,  209,  230,  231,  262, 

263,  303 

Parasites,  64,  68,  216,  244,  245,  350 
Parenchyma,  II,  77,  81 
Parmelia,  233 
Parnassia,  448 
Pasteur,  164 
Pea,  family,  452 

sweet,  453 
Peach  curl,  248 
Pear,  branch  of,  73 
Peat,  298 

moss,  296 
Pedicel,  403 
Pediastrum,  184 
Penicillium,  217,  228,  242 
Perennials,  69 
Perianth,  115,  116,  286,  287,  288,  379, 

383 

Periderm,  266,  267,  268 
Perigynous,  385,  386 
Perisperm,  397 
Perisporiales,  244 
Peristome,  294,  305,  306,  307 
Perithecia,  229 
Peronospora,  220,  223 
Peronosporales,  220 
Personate  flower,  478 
Petals,  115,  116,  384 
Petiole,  7,  8,  29,  42,  43,   107,  426, 

427 

Peziza,  231 
Pezizales,  229 
Phaeophyceae,  201 
Phallales,  268 
Phallus,  268 
Phegopteris,  330 
Philotria,  2 

Phloem,  59,  79,  91,  314-315 
Phosphorescence,  259 
Phlox  order,  474 
Phosphorus,  16 
Photosynthesis,  11-16,  23 


502 


INDEX 


Photosynthesis,  economic  importance 

of,  15-16 
Phycocyanin,  165 
Phycomycetes,  218 
Phyllactinia,  247 
Phylloglossum,  343,  344 
Phytophthora,  221,  223 
Picea,  375,  376 
Pierce,  190 
Pigments,  32 
Pileus,  260 
Pilobolus,  226 
Pinales,  362 

classification  of,  375 
,/Fine,  365,  366,  368,  369,  371,  372,  374, 

375 

wood,  94,  363 
Pinguicula,  480 
Pinnularia,  169,  172 
Pistil,  115,  116,  399 
Pit,  398 

Pith,  76,  78,  314-315,  324,  342 
Placenta,  388,  389 

free  central,  474 
Plankton,  168,  169 
Planktoniella,  169 
Plant,  nature  of,  I 

cell,  I 

Plasmodium,  151,  152 
Plasmopara,  221,  222 
Plastids,  2,  17 
Pleurococcus,  182 
Plowrightia,  239,  240 
Plumule,  125,  126,  398,  428 
Pod,  127,  130,  398,  451,  454,  455 
Poison  ivy,  459,  460 

sumac,  459,  460 
Poisonous  secretions,  39,  40,  53,  265, 

442,  459,  469,  481 
Polar  nucleus,  118,  119,  389 
Polemoniales,  474 
Pollen  grains,  116,  I2O,  122 
Pollinium,  424,  425,  474 
Polygona'es,  437 
Polypodium,  329 
Polyporaceae,  264 
Polysiphonia,  212 
Polystichum,  311,  322,  328 


Polytrichum,  302,  303,  306 

Pond  scum,  185 

Poplar,  leaf  arrangement,  28 

order,  429 
Poppy,  family,  442 

order,  442 

Pore,  n,  80,  8 1,  94,  264,  265,  283,  363 
Porella,  289 
Potato,  481 

rot,  221 

Prepotency,  of  foreign  spores,  454 
Primitive  plants,  46,  154 
Primrose  order,  473 
Primulales,  473 

Pro-embryo,  359,  360,  371,  395,  396 
Prosenchyma,  II 
Protandrous,  283 
Prostrate  stems,  109 
Protection  of  microspores,  412 
Proteids,  16 
Proteid  grains,  18,  19 
Prothallium,  312,  317,  330,  331,  370 
Protogynous,  283 
Protomyxa,  153 
Protonema,  308,  309 
Protoplasm,  function  of,  3 

nature  of,  2,  3 

Protoplasmic  connection  of  cells,  3 
Pruning,  100,  102 
Prunus,  451 
Pteridophyta,  311 
Pteridium,  329 
Ptomaines,  159 
Puccinia,  251,  252,  253,  255 
Puffball,  266,  267 
Pulvinus,  35 

Purposive  reactions,  5,  36,  104 
Pussy  willow,  430 
Pustules,  258 
Pycnium,  252 

Pycnidium,  pycnidia,  233,  252 
Pycnidiospores,  252 
Pyrenoids,  185,  186 
Pyrola,  384,  470,  471 
Pyronema,  229,  230 

Quarter-sawed  oak,  95 
Quercus,  146,  383,  434 


INDEX 


503 


Ragweed  family,  485,  489 
Ranales,  438,  440 
Ranunculus,  438,  439 
Ray,  flower,  489,  490 

wood,  92,  93,  94,  314,  315,  363 
Rattlebox,  yellow,  479 
Recessive  factor,  143 
Receptacle,  381-386,  439 
Red  algae,  210,  218 

bud,  451 

snow,  176 

Regeneration  of  stems,  364 
Respiration,  8,  21,  54,  88 
Reproduction,  no,  136 
Resin,  39,  364 
Resurrection  plant,  346 
Rhamnales,  461 
Rhinanthus,  479 
Rhizoids,  269,  273 
Rhizomes,  no,  in,  323,  324 
Rhizopus,  224,  225,  226 
Rhododendron,  leaf,  41 
Rhodophyceae,  210,  211 
Rhus,  460 
Ribes,  449 
Riccia,  272 
Ricciocarpus.  272,  273,  274,  276,  278, 

281 

Richards,  75 

Ring,  annual,  73,  75,  91,  92 
Rivularia,  166 
Rock,  decomposition  of,  49 

disintegration  of,  49 
Rockweed,  206 
Root,  i,  4,  46,  298,  314 

aerial,  64 

anchoring  and  supporting,  69 

bacteria  on,  65 

bending  of,  62,  63 

binding  action,  70 

cap,  58,  60,  398 

contraction  of,  70 

growth  of,  47,  56-58,  63 

hairs,  46,  47,  52 

lateral,  59,  62,  63 

modification  of,  64,  109 

sensitiveness  of,  61,  109 

storage  organs  of,  69 


Root,  structure  of,  56,  59,  60,  61 

surface  of,  55 

toxic  substances  from,  53 
Rootstock,  1 10,  in 
Resales,  447 
Rosette,  29 
Rose,  385 

family,  450 

order,  447 
Rostellum,  423,  425 
Rubiales,  481 
Rush,  401,  416,  418 
Rusts,  251 

white,  220,  223 

Saccharomycetes,  248,  249 
Sac  fungi,  228 
Sage,  477 
Salicales,  429 
Salix,  381,  430 
Salvia,  477 
Saltpeter,  66,  160 
Samara,  398,  458,  459 
Sambucus,  481,  482 
Sanguinaria,  442,  443 
Sap,  3,  33,  69,  71,  83 
Sapindales,  456 
Saprolegnia,  219,  222,  223 
Saprolegniales,  218 
Saprophytes,  153,  217 
Sapwood,  95 
Sargassum,  206 
Saxifrage,  387,  449 
Scabiosa,  481,  482 
Scenedesmus,  182 
Schizocarp,  458,  459 
Schizophyta,  154 
Scitaminales,  420 
Sclerenchyma,  78 
Sclerotium,  237,  238 
Scouring  rush,  336 
Scrophulariaceae,  478 
Scutellaria,  477 
Scutellum,  135,  409 
Secondary  growth,  85 
Sedge  family,  410 

order,  405 
Sedum,  116,  379,  448 


504 


INDEX 


Seed,  351,  352,  360,  372,  397,  398 

development  of,  125,  126,  130 

forms  of,  126,  127 

functions  of,  127 

growth  of,  127-136 
Seedling,  castor  bean,  132,  133 

corn,  134,  135 
Seed  plant,  352 
Segregation  of  factors,  141 
Selaginella,  346,  347,  348,  350 
Selaginellaceae,  346 
Senna  family,  450 
Sensitive  pea,  451,  452 

plant,  35,  36,  455 
Sensitiveness  of  leaf,  33 

of  protoplasm,  4,  5,  152 

of  root,  6 1 

of  stem,  104,  106 

of  zoospores,  176 
Sepals,  384,  441 
Seta,  289,  291,  309 
Sex,  evolution  of,  178,  180,  181,  182, 

194,  198,  204 

Sexual  generation,  136,  137,  279 
Sexuality,  nature  of,  171,  172,   177, 

181,  185,  204 
Shade  plants,  41,  44 
Shin  leaf,  470,  471  y 

Sieve  tubes,  79,  80 
Silicates,  48,  170,  173,  336 
Silk,  of  corn,  134 
Siphonales,  195 
Skull-cap,  477 
Skunk  cabbage,  412 
Sleep  movements,  35 
Slime  mould,  149,  150,  151 
Smallpox,  163,  164 
Smuts,  257 
Soapberry  order,  456 
Soils,  care  of,  54,  55 

formation  of,  48-50 

nature  of,  50-52 

retention  of  substances  by,  52 

sterility  of,  53-55 

structure  of,  51 

texture  of,  51 
Solanum,  481 
Soldier's  cap,  443,  444 


Solomon's  seal,  leaf,  7,  401 
Soredium,  234,  235 
Sorus,  316,  324.  325,  355 
Spadix,  411 
Spathe,  410,  411 
Spathyema,  412 

leaf  structure,  41 
Species,  146 

biological,  245 
Sperm,  196,  275 
Spermatophyta,  352 
Sphaerella,  174,  175 
Sphaeriales,  239,  240 
Sphagnales,  296 
Sphagnum,  296,  297,  299,  300 
Spindle,  of  cell,  57,  58 
Spiral  leaf  arrangement,  27,  28 
Spirillum,  155 
Spirogyra,  185,  186,  187 
Spleenwort,  329 
Spongy  mesophyll,  9,  II,  21 
Sporangia,   117,   150,   192,  224,  225, 

315,  325.  326 
Spore,  115,  118,  151,  270 

differentiation  of,  339 

germination   of,    278,   300,    301, 

330,  331,  338,  349 
Sporophylls,  115,  116,  117,  337,  338, 

379,  38i 
Sporophyte  generation,  136,  194,  215, 

256,  281 

Sporophyte,    development    of,    281, 
286,  291,  293,  304,  310,  312,  318, 
319,  332-334,  346,  359,  371,  395, 
396,  399 
Squash,  leaf,  10 
stem,  79 r  80 

Stamen,  115,  116,  379,  399 
Staminodia,  421,  473 
Standard,  451-453 
Starch,  2,  12,  17,  18,  19 

test  for,  14 
Staurastrum,  188 
Steironema,  473,  474 
Stem,  branches  of,  99,  100 
dicotyledon,  84,  90,  93 
function  of,  71 
growth  of,  89, 90, 91 , 92, 93, 98, 99 


INDEX 


505 


Stem,  healing  of,  101 
modification  of,  105 
monocotyledons,  84,  96,  400 
sensitiveness  of,  104 
strengthening  tissues  of,  83 
structure  of,  71,  73,  77,  90,  91, 

92,  93 

Stemonitis,  150 

Stereome,  37,  38,  39,  78,  79,  84,  85, 

323,  325 

Stereum,  264 

Sticta,  233 

Stigma,  116,  117,  380 

Stimuli,  nature  and  effects  of,  4 

reaction  to,  5,  33-36,  40,  61-63, 

104,  106,  121 
Stink  horns,  268 
Stipe,  260 
Stipules,  426 
Stolon,  109,  no 

Stoma,  8,  9,  10,  21,  25,  26,  37,  88,  293 
Stone  fruit,  398 
Storage  organs,  18,  59,  77,  no,  in, 

"3,  132 

Strawberry,  384,  450 
Streaming  of  protoplasm,  151,  152 
Strobilus,   333,   337,   344.   347,   353, 

365,  366,  374 
Stroma,  238 
Style,  116,  117,  380 
Suber,  86 
Subdivision,  147 
Subkingdom,  148 
Sugar,  12,  16,  17,  120 
Sulphur,  16 

bacteria,  161 
Surface  reductions,  44 

tension,  51 
Suspensor,  124,  125,  345,  346,  360, 

371,  395,  396 
Symbiosis,  68,  234 
Sympetalae,  469 
Symphytum,  385,  475,  476 
Synergid,  118,  119,  120,  390,  391 
Synthesis,  159,  160,  162 

Tabellaria,  168 
Tapetum,  379 


Taraxacum,  485,  486 

Taxodium,  376 

Taxus,  376 

Teeth,  of  peristome,  307.  308 

Telial  stage,  254,  255 

Teliospore  stage,  255 

Teliospores,  254,  255 

Tendrils,  107,  108 

Tetraspores,  213 

Thallophyta,  classification  of,  149 

Thallus,  149,  270 

Thaxter,  153 

Thelephoraceae,  263 

Thistle  family,  485,  489 

Thuja,  376 

Tissue,  strengthening,  83 

Toadflax,  478,  479 

Toadstools,  259,  260 

Toxins,  158,  162,  163 

Tracheids,  9,  81,  94,  363 

Tradescantia,  stoma,  38 

Transpiration,  8,  24,  35 

Transport  of  water,  59,  ^9,  J34 

Tree  ferns,  325 

Truffles,  243 

Tsuga,  375 

Tube,  cell,   121,  122,  358,   369-371 

392,  393 

Tubercles,  65,  161 
Tuberculosis,  163 
Tubers,  in 
Turgor,  83 
Twigs,  73 

Twining  stems,  106 
Typha,  381,  401,  403 

Ulothrix,  190,  191,  193 
Umbel,  466 
Umbellales,  465 
Uncinula,  245,  247 
Unilocular  pistil,  473 
Uredinial  stage,  254,  255 
Uredinales,  251 
Urediniospores,  254,  255 
Urticales,  434,  436 
Ustilaginales,  257 
Ustilago,  258 
Utricularia,  480 


506 


INDEX 


Vaccinium,  472 

Vacuoles,  I,  2,  3,  60,  217 

Valerian,  483 

Valves,  of  diatoms,  170 

Vapor,  from  plants,  24 

Variation,   105,   144,   176,   190,   197, 

218,  280,  312-313,  320,  388 
Variegated  plants,  14,  32 
Vascular  bundles,  n,  12,  16,  76,  77, 

78,  79,  80,  97,  312,  314,  323,  324, 

337,  342,  354 
Vaucheria,  195,  196 
Veins,  7,  9,  n,  301,  302,  312 
Velamen,  422 
Veltheimia,  402 
Velum,  260,  261 
Venation,  7,  400,  401,  426,  427 
Veratrum,  416 
Vernonia,  491 
Versatile  anthers,  408 
Vessels,  79,  80 
Viburnum,  481,  482 
Vitis,  461 
Volva,  261 
Volvocales,  174 
Volvox,  1 80,  181 

Wallace's  Law,  423 

Wall  cells,  277,  318,  358,  392 

Walnut  order,  433 

Ward,  24 

Water-bloom,  165 

Water,  action  on  rock,  49 

capillary,  50 

composition  of,  12 

gravitational,  50 

hygroscopic,  50 

lily,  398,  438 

mould,  218 

net,  184 


Watering  of  plants,  55 
Wax  of  epidermis,  37,  39 
Weeds,  445 
Willow,  381,  430 

order,  429 

Wilting,  cause  of,  24 
Wind,  pollination  by,  388,  404,  433 

458 

Winged  fruits,  373,  458 
Wings,  of  flowers,  451-453 
Witches'  broom,  256 
Wolffia,  410 
Wood,  heart,  95 

oak,  95,  427 

pine,  363 

sap,  95 

sections,  93,  94,  95 

spring,  91,  92,  95 

summer,  91,  92 
Woodsia,  328 
Woodwardia,  326,  328 
Work  of  plants,  15 
Working  of  ponds,  165 

Xanthidium,  188 

Xerophytes,  39,  45 

Xylaria,  239,  241 

Xylem,  59,  76,  78,  79,  91,  3M-3I5 

Yamanouchi,  215 
Yeast,  248,  249 
Yucca,  419 

Zamia,  353,  354,  355,  356 
Zooglea,  155 
Zoospore,  151,  152 
Zygnema,  185,  186 
Zygnematales,  185 
Zygomorphic,  387 


